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

bits 6-0 contain the data to be sent to the track
bit 7 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 01111111 01111111)
(packets beginning with some other sequence are reserved for non-NMRA messages)

Maximum NMRA message length = 16 bytes (i.e. 112 bits).
Messages longer than this will cause things to get confused at the moment.

baud rate 9600
most significant bit first
8n1