NMRA packets from the computer to the PIC should consist of 8-bit bytes.

bits 7-1 contain the data to be sent to the track
bit 0 is 1 if the byte is the last in the packet and 0 otherwise
 
The first 14 data bits in the NMRA packet should be 1s.
(i.e. the first two complete bytes should be 11111110 11111110)
(packets beginning with some other sequence are reserved for non-NMRA messages)

baud rate 9600
most significant bit first
8n1