Dual tunnels
OpenTTD Manual
Installation · Tutorial
FAQ · Game Interface
Readme.txt
Railway Junctions
Junction Criteria
3-Way junctions
Basic 3-Way
Line Merge
Half Cloverleaf
Half Spaghetti
Compact 3-Way
Compact 3-Way B-M Junction
Ultimate 3-way
Half Transmogrified
Braided Junction
4-Way junctions
Basic 4-Way
Roundabout
Cloverleaf
Spaghetti
Star
Complex Star
Branch-Merge
Tetrathorp
High Speed 4-Way Fly-over∕under
Pre-Signaled Roundabout
Advanced Roundabout
Right-of-way Roundabout
Improved Roundabout
Transmogrified
Tightlong
Deepblue2k8 4-way Junction
Enormous junctions
Multi-way/multi-track cloverleaves
Dual Tetrathorp
Dual Transmogrified
Dual Branch-Merge
Quad Branch-Merge
Quad Tetrathorp
6-tuple Branch-Merge
8-Way Star
High Speed 4-way
Other junctions
Building depots
Dual tunnels
Sideline U Turn
Right-of-way Lane Change
Priority Merge
Track Layouts
4-Way Triple Axial
4-Way Triple Axial 2
Triple Via Mini
High Hill
Unuseful junctions
Crossover
3/4ths junction
Ultimate 3-way junction for 4 tracks

Sometimes you need to build long Tunnelss on your mainline. Tunnels do not have signals, so only one train can be inside at a time. This can cause traffic slowdown or jams on your main line.

Dual tunnels

Adding additional tunnels solves this problem. If one tunnel has a train in it when another train arrives, the 2nd train can continue through the 2nd tunnel instead of stopping.

Note the use of Signals to ensure that if both tunnels are in use, trains wait at the junction for the first available tunnel.