as these cannot pass on the junctions due to the short section of single line running.
Clarifying to make sure I have it straight:
(Note: using Au / UK layouts with up on left and down on right)
Double Junction (with or without ladder)
- Train A-E will block D-B, but can still operate F-B
- Train D-B will block A-E (and obviously F-B), but can still operate A-C
Thus with a double junction, it is always possible to operate two trains (one in each direction) through the junction, although the first route may limit the second.
Single Lead Junction
- Train A-E will block D-B and F-B
- Train D-B will block A-E, but can still operate F-B
With a single lead junction, any operation involving A-E will block all lines (and A-E will be blocked by any other operation).
Flyover
A flyover looks schematically like a double junction, except that A-E and D-B do not intersect. Both lines can be fully utilised, because A/C/E do not interact with B/D/F.
Example
Three trains approach the junction: A going A-E, D going D-B, and F going F-B.
- Double junction: A-E and F-B can proceed, D-B waits
- Single junction: only one of A-E, D-B and F-B can proceed at a time, 2 trains wait
- Flyover: A-E can proceed, as can either of D-B and F-B
Collision risks
It looks to me that a ladder junction is technically slightly less safe than a double junction. The double junction has two possibilities of collision:
- Merge between D-B and F-B
- Crossing between A-E and D-B
The ladder junction offers a third possibility of failure:
- Train from A running onto D or train from D running onto A
This third possibility is interesting because it allows a collision between a single train moving through the junction and a stationary train not in the junction. This does require a signalling / turnout failure and not just SPAD.
The single lead junction introduces the additional risk that a train going A-E could collide with a train going F-B.
Flyovers reduce the collision risk to the single D-B and F-B merge.