The last time I used Basemaps was in TRS2006 but I do recall that I was able to take
- a screenshot of an area 1km x 1km from Google Maps,
- use content manager (or whatever it was called back then) yo add it into a Basemapz asset as a .JPEG,
- edit the config.txt file to refer to the image,
- commit the edited asset and then
- load it into Trainz.
I just did a check of the DLS and the asset I actually used was "1km Basemap" followed by a number or letter as shown below.
<kuid2:267614:100010:2> 1km Basemap 0
<kuid2:267614:100001:2> 1km Basemap 1
<kuid2:267614:100002:2> 1km Basemap 2
<kuid2:267614:100003:2> 1km Basemap 3
<kuid2:267614:100004:2> 1km Basemap 4
<kuid2:267614:100005:2> 1km Basemap 5
<kuid2:267614:100006:2> 1km Basemap 6
<kuid2:267614:100007:2> 1km Basemap 7
<kuid2:267614:100068:2> 1km Basemap 8
<kuid2:267614:100069:2> 1km Basemap 9
<kuid2:267614:100011:2> 1km Basemap A
...and so on to ...
<kuid2:267614:100067:2> 1km Basemap Z
Have you tried these ones instead? The description of each asset contains the instructions.