Here's one way I do it - place a piece of  poster cardboard on a table, curved up towards the back.  Weights behind the reflectors holds it in place.  Lighting is by a fluorescent desk lamp with built in magnifying lens.  The circular lamp is especially suited for this purpose, as it provides and overall soft light.  When actually shooting, the lamp would be much closer to the model.

This is a refined version of my posing method,  using a smaller piece of poster cardboard (bristol board), good for smaller models.  In use the reflectors would be a lot closer to the model, to direct more light on it.

This is how I made the small posing stage.   I made a curved back and braces from corrugated plastic (you can buy this or salvage it from old signs), and glued it together with a hot melt glue gun.   The white poster board was glued on with the same glue.  This stage is an option for smaller models and is easily stored out of the way and does not take up as much desk space when being used.

Here are two of my On30 models photographed using these setups. The shadows are lightened with reflectors.  

You can use colored backgrounds or white, for different effects.   The camera is on a tripod, and the white balance is set for the fluorescent lighting - best to make tests using your camera.   Of course you can use incandescent desk lamps for similar photography, just be sure to set the white balance to incandescent.  Ues the lowest ISO setting for the best quality, and turn on the camera self timer to eliminate vibrations when you press the shutter button.

A SUGGESTION - When you set up a scene to photograph, be sure to check and see if all of the wheels of the rolling stock are on the rails.   It is very easy to set an engine on the tracks and not notice that all of the wheels are on the track!   Unless your intention is to show a derailment, do a quick visual check before shooting.   The voice of experience talking here!