I would always go for a ground texture overlay with a map on top of the DEM terrain. The purpose of UTM tiles is to add detail where needed. For many places of a route, the ground texture map will usually suffice.
TIGER (US census map data) could be an option for such a map overlay. However, it's not supported by TransDEM directly, for good reason. TIGER was used quite a lot in the early days of Trainz because it was the only map option that could be combined with MicroDEM and HOG. But if you look at the specifications of the TIGER map you will find that its map scale is only 1:160,000. Apparently good enough for census but leaving a lot to desire when building a Trainz route in high resolution DEM terrain. Rivers flowing uphill are just one indicator of inadequate accuracy.
Instead, I recommend the USGS 1:24,000 topo map, acquired via WMS, a much alrger scale, compared to TIGER. 1:24,000 will still come out sharp enough when put on 5m Trainz terrain. Unfortunately, our long-standing standard WMS source for the 1:24k topo map, Microsoft Terra Server, later Microsoft Research Maps, was shut down in May. But USGS started its own WMS for the 1:24k topo maps a year ago. There is a little catch here: TransDEM relies heavily on the WMS describing the data it offers correctly, via the so-called meta data. But the WMS meta data from that particular USGS server is partly incorrect. I fixed this for the TransDEM WMS client by adding a specific bypass to it. That bypass was implemented in TransDEM 2.2, the current version, which you will need to successfully access that server.