Why are Class 90s used in pairs?

[zwk500]

Would an extra loco give much extra brake force?

I don't know how much, but I've been told it's a reason for doing so. A loco has much more weight on each wheel than a wagon, so even with fully-fitted train I wouldn't be surprised if it helped. However, given DRS operate single 88s routinely I suspect the braking isn't a major factor.

 

[AndrewE]

I don't know how much, but I've been told it's a reason for doing so. A loco has much more weight on each wheel than a wagon, so even with fully-fitted train I wouldn't be surprised if it helped. However, given DRS operate single 88s routinely I suspect the braking isn't a major factor.

Does it?
A 90 is 84.5 T on 4 axles according to Wikipedia, https://www.google.com/url?sa=t&rct...ns__WAG_.odt&usg=AOvVaw2KPHS4kLEvrxODxzY8Eg5J says

The NTSN shall apply to freight wagons with a maximum operating speed lower than or equal to 160 km/h and a maximum axle load lower than or equal to 25t.​

which I reckon allows a bogie wagon up to 100T.

 

[Nottingham59]

The main gradient Northbound over Shap is, I think, 1 in 76. So a single Class 90, delivering 3.7MW could theoretically pull a 1000T intermodal at 60mph, assuming no losses in the transmission or wind / rolling resistance. The tractive effort required (130kN) is well within the capability of the locomotive.

But in the real world, with rolling resistance, significant wind resistance and auxiliary power load, the uphill speed would be much less than this.* So I can see why they would normally use them in pairs.

* EDIT: Making sensible assumptions about wind resistance, I calculate a single Class 90 at maximum power could haul a 1000T trailing load up Shap at 40mph without losing speed.