Tram signalling

[Halsebee]

Probably a very basic question for the experts, but I know nothing about signalling and have often wondered why "normal/heavy" rail services have comprehensive signalling, but tram systems, which also run on roads and at very high frequency often, can make do with no signalling at all, just driver observation?
I can understand that signalling is needed for most railways, but why can't slow routes run on the same principle as trams i.e. driver observations instead of signals?

 

[jopsuk]

to a certain extent they do- on single line, Radio Electronic Token Block areas between the station loops there are (almost) no signals- nothing protecting, for example, crossings. The signallers don't actually have any way of knowing the location of the train other than the section (which may be quite long) and a bit of guess work assmuing it is running to time.

On short branches where only a single train operates (Corwall, St Albans etc) there is absolutely no signalling after the junction station.

Ideally though there should be, on the main network, just a single signalling type. It is the cost of upgrade that prohibits this.

However, street-running trams don't have a complete lack of signalling. They do however have signals that are designed not to confuse other traffic.

Also, trams are generally much lighter than trains. They have the ability to accelerate and brake faster usually- required when operating in mixed traffic. Even at slow speeds, a heavy goods train can require quite a distance to stop.

 

[Tomnick]

RETB and OTW lines might have minimal signalling (but the lack of signals between loops on an RETB line is no different to the lack of signals between loops on many other single lines!), but they are nevertheless signalled to ensure that a train will never unexpectedly come across another one in front. The Esk Valley and Central Wales lines are slightly unusual in that the Driver of a train arriving at a loop cannot be certain that the loop isn't still occupied by the previous train; the section itself is guaranteed clear though.

The big issue though, as far as these rural low-speed lines are concerned, is that they are usually single-track for the most part. In that case, preventing two opposing movements onto the single line is the key concern, and line-of-sight wouldn't help there!

Even where signalling is provided for trams, I understand that it is primarily to avoid conflict with road traffic at junctions, and other trams on conflicting routes. Separation between two trams following each other (and of course a tram following road traffic!) is maintained largely on line-of-sight.

 

[Crossover]

This is something which amazed me when I was on the Croydon Tram system earlier this year - you could have thought you were on a bus as I think they merely followed the vehicle traffic lights in places and a tram came within mere feet of ours when we were at a station

 

[notadriver]

How do you define a "slow route"? I think Trams are a bit like buses and require less skill to drive than a heavy rail train because they can accelerate and brake like a bus. A conventional passenger train has nowhere near the same sort of braking capabilities.

 

[thelem]

Separation between two trams following each other (and of course a tram following road traffic!) is maintained largely on line-of-sight.

I think that's one of the key differences. It is assumed that a train track will be clear unless it is occupied by another train - therefore there is no need to be able to "stop in the distance that you can see", to quote the highway code. With a tram, you've no idea what obstacles are in the way, so you have to assume that your route is blocked at the limit of your vision.

 

[Metroland]

All goes back to light railway orders, because of speed you don't need such comprehesive safety requirements (full signalling, fencing and so on). Heavy rail is generally over 25mph, and braking distances are such that full signalling is required for train seperation. Because of the mixed traffic nature of heavy rail, signalling is also used for traffic regulation.

Likewise motorways have more traffic signs on than normal roads to warn of potential hazards and junctions.

Some branch lines were built and most tramways were built with Light Railway orders, to save money, most famously the Colonel Stephens lines.

http://en.wikipedia.org/wiki/H._F._Stephens

Some modern tramways have signalling these days at traffic lights to give them priority and out on suburban lines converted from heavy rail as the speeds are greater.

 

[telstarbox]

On the Sheffield Supertram there are some signals to avoid conflict with road traffic at junctions (the tram has a separate phase at some intersections while the traffic lights stay red), but AFAIK the off-road sections such as Fitzalan Square - Meadowhall are mostly line of sight.

 

[The Decapod]

Don't modern trams have emergency electomagnetic track brakes, so if need be they can stop as quickly as a bus, by effectively sticking themselves to the track?

 

[WatcherZero]

Yeah, the emergency brakes will deaccelerate a tram faster than a bus could brake. normal braking about 1.3m/s/s Emergency brakes 2.6m/s/s

 

[Clip]

This is something which amazed me when I was on the Croydon Tram system earlier this year - you could have thought you were on a bus as I think they merely followed the vehicle traffic lights in places and a tram came within mere feet of ours when we were at a station

That would have been in Croydon yes? because when it leaves there and goes through mitcham junction and on to Wimbledon theres definitely proper signals- although not in the National rail sense