Freight Train lengths in the UK are much shorter then those in the US, generally due to weight limits on bridges, and the more compact nature of British Railways. I willtry to give you an idea of some train lengths.
Coal: can be in lengths of up to 37 HAA type hoppers, up to 24 HTA hoppers.
If you search UKTrainsim for these wagons, or visit
http://www.ews.co.uk
or
http://www.freightliner.co.uk you will get a good idea of the capacity of these
trains. Intermodal, Waste, Auto Trains can also vary from a few wagons to over 20. The longest train I have heard of recently was hauled by 5 class 66 locos, of 37 engineers wagons.
Any other questions, just ask.
Your question on loco usage is on the spot. Most UK passenger services are now Multiple Unit based, exceptions being charter and relief trains, and Sleeper services. The HST is basically a high speed DMU. The IC225 which has a 6000hp electric loco pushing/pulling 9 coaches and a DVT was invisaged as running in "Freight" mode at night, but has never done so. Almost any loco can pull Freight or passenger, although in practice a Class 55 "deltic" for example was specifically passenger orientated and wasn't used for freight... Locos are usually "Rostered" to specific tasks, but even the Class 66, a primarily freight engine, can haul passenger trains if needed. (In fact, Empty coaching stock is technically a "freight" train. Other types such as Class 37 for example were "general purpose" although as with many other locos, there use on specific trains depended on things like what type of train heating supply they were fitted with and so on.... a freight only version might have no heating supply so wouldn't be used, or it might have steam heating, limiting the type of coach it could pull, and braking is also an issue. (None of this applies to Empty passenger stock tho....
)
