Why are locomotives so big?

[DynamicSpirit]

Diesel and electric locomotives that pull passenger trains seem to be typically the length of a short carriage - so a pretty large space, which I assume is devoted entirely to the engine plus two driving cabs.

But DMUs have been around since the 1950s and EMUs since the 1890s, and they rather obviously pack the engine into a far smaller space - usually underneath the train. Their existence appears to amply prove that engines don't need to be the size of a whole railway carriage! So why are locomotives that big?

 

[Unobrow]

Diesel and electric locomotives that pull passenger trains seem to be typically the length of a short carriage - so a pretty large space, which I assume is devoted entirely to the engine plus two driving cabs.

But DMUs have been around since the 1950s and EMUs since the 1890s, and they rather obviously pack the engine into a far smaller space - usually underneath the train. Their existence appears to amply prove that engines don't need to be the size of a whole railway carriage! So why are locomotives that big?

The engines do need to be that big, and also having a whacking great generator pack to provide electricity for the traction motors. You won’t get a DMU hauling a 2,400 ton freight train.

 

[bluesfromagun]

Diesel and electric locomotives that pull passenger trains seem to be typically the length of a short carriage - so a pretty large space, which I assume is devoted entirely to the engine plus two driving cabs.

But DMUs have been around since the 1950s and EMUs since the 1890s, and they rather obviously pack the engine into a far smaller space - usually underneath the train. Their existence appears to amply prove that engines don't need to be the size of a whole railway carriage! So why are locomotives that big?

Put very simply, DMUs have a small-ish engine which generally has a mechanical transmission and they move nothing but their own weight around. Like a bus.
Locomotives generally have a much larger engine which generates electricity via an alternator / generator, which power traction motors on the wheels. Locomotives have to move sometimes thousands of tons, dead weight, so they have enormous engines that generate a lot of power just to move whatever they happen to be dragging.
With an engine that large, and they are massive, there's a lot of cooling equipment and electronics to fit in too. Locomotives may look huge outside, but inside they're very cramped due to the sheer volume of equipment they have inside.

 

[WirralLine]

Not to mention your typical DMU, (all DMUs?) have one engine per coach, so a 9 car DMU formation would have 9 engines. Effectively a DMU car carries its own weight.

A loco hauled 9 car has only 1 engine in the loco itself to pull the weight of not only itself but the 9 dead weight coaches too. (Don't be smart and bring 2 engined locos into the mix!)

 

[Wilts Wanderer]

Generally U.K. locomotives are on the small side compared to continental or (particularly) US designs, mainly because of our smaller loading gauge. This translates into somewhat lower power output but also puts stress on the design by cramming everything into a smaller space which can mean reliability suffers. For locos, bigger is usually better!

 

[Mollman]

Diesel and electric locomotives that pull passenger trains seem to be typically the length of a short carriage - so a pretty large space, which I assume is devoted entirely to the engine plus two driving cabs.

But DMUs have been around since the 1950s and EMUs since the 1890s, and they rather obviously pack the engine into a far smaller space - usually underneath the train. Their existence appears to amply prove that engines don't need to be the size of a whole railway carriage! So why are locomotives that big?

Southern region DEMUs (e.g. class 205s) had an engine in one coach only, with the remaining coaches acting as trailers. The engine bay took up around half the coach.

 

[Joey Beauchamp]

Are the Class 755 FLIRT's similar? Looks like they have an engine in the middle.

 

[Peter Mugridge]

Are the Class 755 FLIRT's similar? Looks like they have an engine in the middle.

They have two ( 755/3 ) or four ( 755/4 ) small engines in that short carriage, so yes - similar principle to the old DEMUs.

Bear in mind these are bi-mode which also work off the 25kV overheads where available.

 

[coppercapped]

Diesel and electric locomotives that pull passenger trains seem to be typically the length of a short carriage - so a pretty large space, which I assume is devoted entirely to the engine plus two driving cabs.

But DMUs have been around since the 1950s and EMUs since the 1890s, and they rather obviously pack the engine into a far smaller space - usually underneath the train. Their existence appears to amply prove that engines don't need to be the size of a whole railway carriage! So why are locomotives that big?

Your assumption that the space is devoted entirely to the engine and driving cabs is wrong. Have you ever looked inside a large locomotive or sought out some drawings of the interior layout?

There are some basic concepts in relation to locomotive and train design that are necessary to comprehend. Firstly that the adhesion between the wheels and the rails is limited, typically to about a third (or slightly more) of the weight on the wheel/rail interface. Only this proportion can be converted to tractive effort at the wheel and therefore to pull at the drawbar. A train's acceleration is directly proportional to this force (see Isaac Newton in his Philosophiæ Naturalis Principia Mathematica 1687) and for a given force this is divided among all the driven axles in a multiple unit. A modern four or five coach long multiple unit could have any number of driven axles from a proportion up to all of them. Modern locomotives have either four or six driven axles so the locomotive has to have sufficient mass to permit the available adhesion to generate sufficient pull. Taking the Class 68 as a modern example some 38% the locomotive's mass of 85 tonnes on four axles gives sufficient drawbar pull (317 kilonewtons) to enable a six or eight coach train to accelerate at a rate similar to an emu with many more driven axles.

Basically, a locomotive has to be this heavy in order to obtain a performance similar to modern multiple units.

Secondly, in order to develop the power necessary to match multiple units' performance quite chunky prime movers are needed. A five coach long Class 80X unit has three one thousand horsepower underfloor engines and a 5 coach Voyager has five 750bhp engines for a total of 3000bhp and 3750bhp respectively. All this power has to be concentrated in one prime mover, in the case of the Caterpillar C175 used in the Class 68 this weighs in at just over ten tonnes yielding 3,800bhp. To which has to be added the mass of the alternator, another three or four tonnes.

Modern diesel engines can be up to about 40% efficient in extracting usable work from the fuel which means that the cooling system has to reject some 5,700bhp through the radiators when the prime mover is running at full chat. This needs a lot of space to work reliably, especially in hot summer days.

In addition there are all the other things which a railway locomotive needs. Things such as the exhaust gas treatment kit and exhaust pipes; the traction motor cooling blowers and ducting; the brake gear and compressed air cylinders for the whole train, including the rheostatic brake grids and fans; the power electronics; the signalling electronics; the engine control and monitoring equipment; fuel tanks; batteries and so on. There also needs to be space for the maintenance crews to be able to get to the kit...

To sum up: I am in awe of the designers who can pack all this into a 20 metre long and 2.7 metre wide box which only weighs 85 tonnes.

 

[The exile]

Southern region DEMUs (e.g. class 205s) had an engine in one coach only, with the remaining coaches acting as trailers. The engine bay took up around half the coach.

The Czechs took this one step further - effectively locomotives with passenger accommodation - always looked odd to see a DMU running round its train. The later examples are still around (classes 842, 843 and 854) but tend now to work with driving trailers more in MU fashion. No doubt a few run-rounds still happen knowing CD.

 

[ac6000cw]

This is an annotated drawing of the equipment layout in a twin-engined class 52 'Western' - notice how packed it is inside, with the diesel engines in the middle part and cooling systems plus hydraulic transmissions between engines and cabs (from https://www.rmweb.co.uk/forums/topic/170700-designing-a-locomotive-from-scratch/ ):

A class 47 diagram:

For something modern, this is copied from the Stadler datasheet for the class 68 (large diesel engine in the middle, with fuel tank hanging below it between the bogies):



...and the class 88 bi-mode datasheet (small diesel engine and cooling system on the right):

 

[Iskra]

Internationally of course, you get pairs of dual locomotives for extra power for heavy freight trains. In the photograph attached, you can see a pair of pairs!

Photo shows a pair of Ukrainian locomotives at Lviv

 

[35B]

The engines do need to be that big, and also having a whacking great generator pack to provide electricity for the traction motors. You won’t get a DMU hauling a 2,400 ton freight train.

There's also a minor point about being able to stop their train - at which point the weight of the locomotive becomes part of its ability to stop a train.

 

[43096]

Even a modern electric locomotive has a lot to pack in, particularly if it either adds diesel or battery capability, or cross-border multi-system functionality.

As an example, this is the internal layout of the Siemens Vectron multi-system loco, with a power rating of 6.4MW. Taken from a Siemens presentation from several years ago (https://www.slideshare.net/slideshow/vectron-innotrans-2010-english/8723762#8).

 

[TRAX]

Diesel and electric locomotives that pull passenger trains seem to be typically the length of a short carriage - so a pretty large space, which I assume is devoted entirely to the engine plus two driving cabs.

But DMUs have been around since the 1950s and EMUs since the 1890s, and they rather obviously pack the engine into a far smaller space - usually underneath the train. Their existence appears to amply prove that engines don't need to be the size of a whole railway carriage! So why are locomotives that big?

Don’t look up North American locomotive weights and dimensions if those in the UK seem too big to you already!

 

[Mugby]

Not forgetting, of course, that many of the 'first generation' diesel locos needed to have space for a steam boiler and consequent water tank.

 

[D365]

Not forgetting, of course, that many of the 'first generation' diesel locos needed to have space for a steam boiler and consequent water tank.

In modern locomotives a similar space is required for Electric Train Supply and/or brake translator equipment.

 

[Jamesrob637]

The engines do need to be that big, and also having a whacking great generator pack to provide electricity for the traction motors. You won’t get a DMU hauling a 2,400 ton freight train.

Ah but in Switzerland an Rhätische Bahn unit will haul freight!

 

[furnessvale]

There's also a minor point about being able to stop their train - at which point the weight of the locomotive becomes part of its ability to stop a train.

However, when the loco gets too heavy, or travels too fast it needs the brakes on the train to help stop it.

Hence the speed limit applied to light locos.

 

[Yorkshire09876]

In modern locomotives a similar space is required for Electric Train Supply and/or brake translator equipment.

Most won't have brake translator equipment fitted. On modern locos, the ETS (if fitted) will use DC-link and won't require a separate ETS generator / alternator as older locos did.

The main user of space compared to on older locos is the exhaust treatment equipment required due to emissions regulations.

 

[Trainman40083]

Brings back memories of Class 40/44/45/46 power units at Derby Loco....I'd wager engines that produce the same power these days are a lot smaller and capable of hauling heavier trains.

 

[ac6000cw]

Modern diesel engines can be up to about 40% efficient in extracting usable work from the fuel which means that the cooling system has to reject some 5,700bhp through the radiators when the prime mover is running at full chat. This needs a lot of space to work reliably, especially in hot summer days.

I agree (although some of the waste heat is carried away in the exhaust gas).

As an example, look at the size of the radiators at the rear of large modern US freight diesels (with 'at the crankshaft' 4800hp engines), which have to deal with even more waste heat. I've seen them running slowly upgrade at full power in dust-laden air (a wheat field was being harvested alongside the track) so you could easily see the air movement, with a column of fast-moving hot air 40+ feet high above each radiator fan, plus the equally tall column of exhaust gas... (my video of it)

== Doublepost prevention - post automatically merged: 4 Jul 2025 ==

Brings back memories of Class 40/44/45/46 power units at Derby Loco....I'd wager engines that produce the same power these days are a lot smaller and capable of hauling heavier trains.

Compared to any of those, the new class 99 six-axle bi-mode is higher power-at-rail on diesel, is somewhat lighter, has 6MW (over 8000hp) available when running on electric, and has roughly twice the low-speed tractive effort of a cl. 45/46.

 

[Dunfanaghy Rd]

Another way of looking at it:
An average 2-car DMU used to have 2 6-cylinder engines producing about 300HP;
A Class 66 has 1 12-cylinder engine producing 3,000HP.
Pat

 

[edwin_m]

There's also a minor point about being able to stop their train - at which point the weight of the locomotive becomes part of its ability to stop a train.

That was quite significant in the days of unfitted freights where only the locomotive and brake van had brakes that could be applied while on the move. Rather less so these days when all wheels of the train are braked, and as mentioned above light engines are speed restricted as their brakes are relatively less effective than those on a train.

Compared to any of those, the new class 99 six-axle bi-mode is higher power-at-rail on diesel, is somewhat lighter, has 6MW (over 8000hp) available when running on electric, and has roughly twice the low-speed tractive effort of a cl. 45/46.

This is at least partly because modern locomotives have "creep control" systems that maximise the friction between the wheel and the rail. The ratio of the tractive effort to the weight on powered wheels is equal to the coefficient of friction that is needed to exert that amount of traction effort. In the case of the 45 and 46 (and the 31 and 40) some of the weight was on unpowered wheels so wasn't contributing to traction.

 

[Deepgreen]

There's also a minor point about being able to stop their train - at which point the weight of the locomotive becomes part of its ability to stop a train.

These days, unfitted trains are rare, and when they do run, they have braked 'dummy' vehicles included for the required brake force.

== Doublepost prevention - post automatically merged: 5 Jul 2025 ==

Diesel and electric locomotives that pull passenger trains seem to be typically the length of a short carriage - so a pretty large space, which I assume is devoted entirely to the engine plus two driving cabs.

But DMUs have been around since the 1950s and EMUs since the 1890s, and they rather obviously pack the engine into a far smaller space - usually underneath the train. Their existence appears to amply prove that engines don't need to be the size of a whole railway carriage! So why are locomotives that big?

EMUs don't have an engine at all - just traction motors and associated equipment.

 

[supervc-10]

This is at least partly because modern locomotives have "creep control" systems that maximise the friction between the wheel and the rail. The ratio of the tractive effort to the weight on powered wheels is equal to the coefficient of friction that is needed to exert that amount of traction effort. In the case of the 45 and 46 (and the 31 and 40) some of the weight was on unpowered wheels so wasn't contributing to traction.

I presume this is the equivalent to Traction Control in a car helping you get off the line in slippery conditions, monitoring the speed of the wheelset and adjusting power/braking to the wheelset very quickly?

These days, unfitted trains are rare, and when they do run, they have braked 'dummy' vehicles included for the required brake force.

And this is the extra coaches that are added when a unit is being dragged, right?

 

[edwin_m]

I presume this is the equivalent to Traction Control in a car helping you get off the line in slippery conditions, monitoring the speed of the wheelset and adjusting power/braking to the wheelset very quickly?

Similar, but it also includes something called creep control which adjusts the wheel speed so it is slipping but only very slightly. The friction of steel on steel is more in that situation compared with no slipping.