Sen City Round Single Small (3 mtr) Tunnel Portal - normal floor.
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 appropriate Sen City Tunnel Bore, (KUID:76656:60109 - on DLS)
3) Cover excess hole with any Retaining Wall, (retainingwall4 by Compudoc, <kuid2:56732:1035:1> is a good match)