From TTWiki

Lists the type of track needed for the vehicle to run.

TRACKST - Standard railroad track
TRACKNG - Narrow Gauge railroad track
ROADTRAM - Tram track
ROADRGH - Rough Road (Eurasian)
ROADONE - One-Way Road (Eurasian)
ROADTMC - Road (Eurasian)
ROADUS1 - Road (US)
ROADUS2 - One-Way Road (US, with yellow line to the left)

Some Vehicle Heads also require a Trackmod, such as
TREXRACK - Rack Rail (for TRACKNG)
TTEXCAT1 - Electric overhead wire (for TRACKST and TRACKNG)
TREX3RL - Electric 3rd Rail (for TRACKST and TRACKNG)
RDEXCAT1 - Electric overhead wire for Trams (for ROADTRAM)