These refer to train targets. (TR)
TRG means Goods "Trucks", i.e. freight cars.
TRP means Passenger Coaches.
Other codes you may see: TRN - complete train with engine, MET mechanical transport.
Here is a link (look at "book" pages 107 - 108)
http://www.scribd.com/doc/87281269/2...NAISSANCE-CODE