Sorry for delay. Finally finished with adding tiles for PK mod.
Here i made example how it can look after changes.
If i understand correctly, "_d" mean diagonal? then If it posible for game to generate railroad which turns from left to right we need both IDs "\" and "/" for diagonals.
May be i dont understant how all stuff work exacly, may be it possible to set tile angle at map preset/generation stage and with Rotation=true flag it will rotates where you need?
But if its not possible, here IDs we should have:
Railroad ties:
t_railroad_tie; it cover both "h" and "v" inner tie tiles because there 2 connected tiles. It also cover outer piece of tie because its just square in my case.
(It posible to make tie fully connected, like this
it will cost for additional ID for each direction
t_railroad_tie_end_north;
t_railroad_tie_end_south;
t_railroad_tie_end_east;
t_railroad_tie_end_west;
not sure if it worth it)
t_railroad_tie_d1; for diagonal /
t_railroad_tie_d2; for diagonal \
I cant figure out how to make diagonal ties solid 
Railroad tracks:
t_railroad_track; it cover both "h" and "v" tiles because there 2 connected tiles.)
t_railroad_track_d1; for diagonal /
t_railroad_track_d2; for diagonal \
Railroad track on ties:
t_railroad_track_h_on_tie; horsiontal
t_railroad_track_v_on_tie; vertical
t_railroad_track_d1_on_tie; diagonal /
t_railroad_track_d2_on_tie; diagonal \
Some map variation, not much better, but a little more smooth.
In this case we can get rid of both these IDs
t_railroad_track_d1_on_tie; for diagonal /
t_railroad_track_d2_on_tie; for diagonal \
Im hope there is a way to make all this more simple.
