Here's an updated version of this set:
https://dl.dropboxusercontent.com/u/61716/BritishTrains.rarYou'll find a wider range of trains, as well as some graphical fixes for those already present. Once again, both Standard and Experimental versions are provided along with sources.
I've also started to experiment with a larger size for some of the smaller trains. Two- and three-car multiple units look a touch too tiny at length=8, so I've started to make them length=10 instead. Here's an example, the East Midlands Trains class 156:

I eventually plan to roll out this change to other units too.
There's still a lot of variance in quality here. Some vehicles, like the HSTs, are more or less up to pakset standard graphics-wise, but others are far from up to scratch at the moment.
Here's a full list of trains included in this set.
Class 43 ('HST'): East Midlands Trains, First Great Western
Class 90: Greater Anglia
Class 142: Northern and Arriva Trains Wales
Class 143: First Great Western
Class 150: Arriva Trains Wales, First Great Western, London Midland
Class 156: East Midlands Trains, Northern, Greater Anglia
Class 158: Arriva Trains Wales, First Great Western, East Midlands Trains
Class 159: South West Trains
Class 165: Chiltern
Class 166: First Great Western
Class 170: London Midland, CrossCountry, Greater Anglia
Class 171: Southern
Class 172: Chiltern, London Midland, London Overground
Class 175: Arriva Trains Wales
Class 221 ('Voyager'): Virgin, Crosscountry
Class 222 ('Meridian'): East Midlands Trains
Class 313: First Capital Connect, Southern
Class 315: Greater Anglia
Class 317: Greater Anglia
Class 319: First Capital Connect
Class 321: Greater Anglia
Class 323: London Midland, Northern
Class 332: Heathrow Express
Class 350: London Midland
Class 357: C2C
Class 360: Greater Anglia, Heathrow Connect
Class 365: First Capital Connect
Class 375: South Eastern
Class 376: South Eastern
Class 377: Southern
Class 378: London Overground
Class 379: Greater Anglia
Class 390 ('Pendolino'): Virgin
Class 395 ('Javelin'): South Eastern
Class 444: South West Trains
Class 450: South West Trains
Class 455: South West Trains, Southern
Class 465: South Eastern
Class 466: South Eastern
Class 507: Merseyrail
London Underground Tube Train (Generic)
M5000 Tram, Manchester Metrolink