In partnership with UK Branch Lines Lesto Route Works has great British Steam Locos
The Cowboys Loco Works offers a few great American style Locos and Rolling Stock including the Union Pacific 9000
Rail Sim provides a few British Locos including the BR Jinty
Quality Payware content, Paulz Trainz offers everything from Mutliple Units to Street Trams if you can afford the expense that is.
Note: Si3D is currently not being updated as frequently as they are now making models for Si3D 2009/Trainz 2009
From the people who bring you the Thomas 2006 content now it's avaliable for Trainz 2009. A must have for Thomas fans. Videos of the models in use can be found on the TrainzContentHelp Youtube Channel.
#1TFM offer mainly content around Thomas the Tank but also scenery items to download. They are available in Standard Gauge and also Narrow Gauge.
This site offers a lot of content based on scratch built Thomas models or reskins (with a twist), but also provides British full scale content.
Sodor Workshops has some Thomas content but it mainly has Thomas reskins, still worth looking on though.
Reskinned and scratch-built Thomas/Non Thomas Content. Including routes and scenery items.
A website which has many interesting routes including the Sodor Steamworks and the Sodor Depot.
Content for Trainz and MSTS, AR3D is managed by AlexTrainz09 and gives great content of reskins and sounds, as well as routes and full scale locos.
Trainz Stuff Online makes mainly routes although they may move to making loco skins in the future.
Sirgibby's TrainZone provides great Narrow Gauge content. It even includes the Ffestiniog Railway and Puffing Billy Railway Content
Pages crammed full of content based around the narrow gauge scale track of 24 inches
Tom has a love for narrow gauge and creates content for 24 inch track, he was a creator for 24ng so he knows what he's doing
This site provides great old British Buses for Trainz
Andi06 provides brilliant old British buses, double and single as well as other great content as well including London Tube Stock and the DLR Stock.
LTSV Trainz has excellent Car and Bus models that run in static, traffic or drivable mode.
This website has a large selection of British Steam and Diesel Locos to download as well as a few routes
Mark's Trainz Site gives excellent Diesel Multiple Unit Locos including First Great Western's 150 Class
Birch Island Railway Workshops makes brilliant sessions, routes and rolling stock. There are also a collection of loadable items.
A recently made website for Thomas Trainz routes and reskins, made by No1thomasfan2010 and ThomasPercy99
A little the same to North Western Railway 3D, Train4475 3D brings you fantasic scratch-built and reskinned Thomas models.
American styled narrow gauge locos and rolling stock for 30 gauge track