Sen City Round Double Small (3 mtr) Tunnel Portal - High Floor. Removes track gravel in tunnel. Better with some track than others.
Also, this portal has the option to automatically change camera views when train in focus enters a tunnel. Not needed if you are using true tunnel track with 'istunnel' flag set.
Instructions...
1) Use DigHole asset to make hole in terrain.
2) Place Portal and attach connecting track and 2x appropriate Sen City Tunnel Bores, (KUID:76656:60109 - on DLS)
3) Cover excess hole with any Retaining Wall, (retainingwall4 by Compudoc, <kuid2:56732:1035:1> is a good match)