Branch-Merge junctions are twisting arrangements of track that prevent any two trains, coming from different directions and heading in different directions, from getting in each other's way if they hit the junction at the same time. They take up more space than simpler junctions, but can't deadlock as no loops are involved (notwithstanding larger loops in the network).

Simplistic junction design is thwarted by:

• Simplistic signalling — Signals and trains aren't smart enough to determine that they won't cross each other's path in a shared signal space, so one will wait for the other unnecessarily. It's also hard to get trains to wait before crossing a genuinely shared piece of track until they know they can clear it.

• Turns — Sharp turns reduce the maximum speed permitted on a route through the junction, and so increase its transit time.

• Hills — Hills caused by tunnels and bridges also reduce maximum speed.

• Bridges — Signals are not (yet) possible on bridges, so long bridges delay rapid successions of trains. Bridges have speed limits, and the very shortest bridges cannot be upgraded to faster ones. Ideally, a bridge should be four tiles long.

• Tunnels — Signals are not (yet) possible in tunnels, so long tunnels delay rapid successions of trains. Preferably, no tunnel should be more than four tiles long, or there should be redundant tracks.

• Space — Junctions often have to fit between towns and industries, and between hills, lakes and valleys.

• Breakdowns — Even if main lines are replicated to allow faster trains to overtake slower (or broken-down) ones, a route across a junction with only a single path can become congested.

The challenge for all junction design is therefore to be:

• smooth (few hills and bends),
• compact,
• fast (low transit time due to limited use of tunnels and bridges),
• replicated (having redundant paths on each route).

Tetrathorps allow forward routes to go straight. Stars have rotational symmetry.

It's almost inevitable that some line will be busier than the others, and the exit onto that line will become a bottleneck. Instead of merging the three tracks from the other lines onto that line's track, why not keep them separate, but allow them to cross over each other?

This increases the line's capacity, and allows it to cope with breakdowns better.

However, there is only one track from any source line to any destination line, so popular routes still risk congestion. What is really needed is a separate junction per track. (Something like this!)

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