Return Cutoffs . How to turn an engine without it leaving the track. . Yes, yes, I know it's a lot of reading... so unless you're a train enthusiast like myself, you probably won't want too. Or you could just look at the pretty pictures. Anyway:
A common complaint among 9V train users is the inability to turn an engine without picking it up or using a turntable. As turntables require ramps or an elevated layout, and precise alignment to avoid derailment; I present the solution of a return cutoff: a simple loop of track that goes back the way it came.
Unfortunately, as Lego® tells us itself, a loop of track as such will cause a short circuit and the train will not run.
Lego® switches, when supplied with current from the single end; provide current to both outside rails of the double end, as well as to one inside rail, the one towards which the switch is set. This means that the fourth rail, the inside rail towards which the switch is not set, will be “dead”, and may accept either charge from that direction.
To avoid a short circuit, a pair of switches can be used to create a separate block inside the loop, so that current is only applied from one side at a time. As long as at least one switch is set towards the outside of the loop, opposite charges will not meet on the same rail, and no short circuit will occur.
Following the diagram in the main image, in which current is supplied from the single end of the loop switch (at the bottom); the train approaches the loop with the block switches set to the left and the loop switch set to the right. The train enters the loop and goes all the way past the first and second switches, but does not reach the third. The train then stops and all switches are changed. This will disconnect the current from one side of the loop block and reconnect it on the other. Next, turning the regulator knob in the opposite direction from which it just came, the train will continue forwards and leave the loop reversed.
If a short circuit does occur using this method, be sure that all switches are in the correct positions and that the train motor is not bridging the gap at either end of the loop block. (This occurs when the motor is at the very base of a switch, so the current is carried through the wheels and onto what should be a disconnected rail). A short circuit will not damage motors, tracks, or electronics; but will not allow trains to run as long as it exists.
When designing layouts using one or more return cutoffs, it is generally a good idea to figure out on paper or with a computer which combinations of switches will result in short circuits and which will not. As a rule, layouts in which the block switches are used as sidings and are not connected elsewhere; and layouts with fewer return cutoffs overall will be easier to run than those with more connections and more return cutoffs. A simple layout employing this method can be created by making a mainline with one return cutoff at each end, allowing a train to drive back and forth along it without backing up. Other combinations of spurs and sidings can be added between the return cutoffs, as long as the block switches remain unconnected. (Layouts with connected block switches are possible, but confusing; and frankly not very much fun to run).
A sample layout combining several return cutoffs. When the appropriate switches are set in the direction of the red arrows, all track except the "X" in the center is usable; and when they are set in the direction of the blue arrows the figure-eight and outermost loop are usable. So, as long as the train is in an area accessible to both sets before the switches are changed it will continue to run and no short circuit will occur. A switch with no red arrow may remain in either position as long as the red set is in use; and likewise for the blue set. The yellow track piece near the center is the one to which current should be supplied.
To avoid confusion, switches can be discretely labeled using coloured 1x1 round plates.
Yes, the diagrams were drawn in Paint. Leave me alone.