So many ideas....so here we go.....
As my Colombian HVAC Foreman would say "we're gonna try something crazy here".....
Hear me out Mark. I like where the discussion is going.
First suggestion is to split this up. Have the road out of town take a slightly more gentle down slope. Then split it so that the lower road is close to the track heading up with a right turn at the end into the tunnel below the track. The road can be surrounded by retaining walls on both sides.
THEN.....the lower road could S turn from the other side of the switch (left to right) and tie into the other road going into the tunnel.
I envision the upper level road bridge over the gap being a stone arch bridge.
Just my $.02......
Bob