Dual tunnels

From OpenTTD

Railway junctions 3-Way junctions Basic 3-Way Line Merge Half Cloverleaf Half Spaghetti Compact 3-Way Compact 3-Way B-M Junction Ultimate 3-way 4-Way junctions Basic 4-Way Roundabout Cloverleaf Spaghetti Star Complex Star Branch-Merge Over-Under Tetrathorp Pre-Signaled Roundabout Advanced Roundabout Right-of-way Roundabout Enormous junctions Double Cloverleaf Dual Tetrathorp Dual Branch-Merge Quad Branch-Merge 8-Way Star Other junctions Right-of-way Depot Dual tunnels Right-of-way Lane Change Unuseful juctions Crossover 3/4ths junction Ultimate 3-way junction for 4 tracks

Sometimes you need to build long tunnels 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.

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