It's not quite clear to me what your second screenshot shows.
There are two methods in TransDEM of bringing image data to Trainz: ground textures and UTM tiles.
Ground textures are limited to a resolution of 1 pixel per ground vertex, i.e 1 pixel every 5 or 10m, depending on baseboard type. That's good for a map scale up to 1:25,000 or 1:50,000. Furthermore, there is a colour limitation. As the ground textures are being reused, its colours are predefined and come from a palette. The standard one has 20 colours and is good for cartography, it also has 10 m grid lines. The other is a custom set with 128 colours, suitable for background orthimages. It does not have grid lines.
Everything with a larger scale, especially orthoimages as a building aid, should go to UTM tiles, big but otherwise ordinary Trainz assets, mass-produced by TransDEM. With UTM tiles, the maximum theoretical resolution is around 1 pixel per 10cm, with 500 m tiles and and 4096 pixel texture size. However, the Trainz rendering engine will compress the texture and you will not get the full resolution in Surveyor. TransDEM automatically assigns texture size, up the the maximum allowed and depending on the images loaded for a particular UTM square in the TransDEM window. With UTM tiles, there is no colour limitation for the textures, at least theoretically, as the Trainz rendering engine appears to make adjustments here, too.
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