Auto bridge

[railfanespee4449]

I want a layout that has crossing tracks, but my trains all run at different speeds. Lionel crossovers + different speed trains = CRASH![B)]
So i think if i add Lionel's modern bascule bridge that stops a train if it is open, i could bridge the tracks without raising the tracks any. The bridge would be slightly elevated, so the moving brridge itself rested on the main line. how could i wire it so a train on the main line raised the bridge, allowing it to proceed, and when it had passed, the bridge lowered so the train on the other track would proceed?

[lionelsoni]

How about this instead? Put a section with an isolated running rail at each of the four sides of the crossing. Use the isolated rails to operate two relays, each cutting power to the center rails for some distance on the conflicting path. So, which ever train gets to within about 10 inches of the crossing first, cuts power for the other train within a yard or so of the crossing, until it has passed. You could even set up signals operated by the relays, in all four directions.

[palallin]

I do not remember which issue, but CTT published a set of wiring instructions for the old MPC drawbridge which may be adaptable to this problem. That bridge was built to be only an inch or two above the other track, so it had to function as you desire. Since the diagram was in the Q & A section, it might be hard to find; perhaps Bob or Jim can help us out here.

[Big_Boy_4005]

QUOTE: Originally posted by lionelsoni

How about this instead? Put a section with an isolated running rail at each of the four sides of the crossing. Use the isolated rails to operate two relays, each cutting power to the center rails for some distance on the conflicting path. So, which ever train gets to within about 10 inches of the crossing first, cuts power for the other train within a yard or so of the crossing, until it has passed. You could even set up signals operated by the relays, in all four directions.

This is exactly how I have done this in the past. Whichever train gets to the crossing first, takes it for itself, and locks out the other path. Just a couple of minor details need to be addressed.

The crossing should have the two paths electrically seperated. Lionel ties all 4 legs of their crossings together underneath. All you have to do is cut the connection for one path, two cuts, and solder an insulated jumper across to create two electrically independent routes.

The other thing is that 10" is too short. 36" is safe. When a speeding train reaches the dead section, it will coast. You don't want to let it reach the crossing, unless you are Gomez Addams.[swg]

[lionelsoni]

I agree with the 36 inches, Elliot. I was thinking of 10 inches as the length of the control section. However, if the train that gets there first only barely does so, the one that is to be stopped may already be too close. So a longer control section. But the controlled section should be longer, so that the second train will stop before it stops the first one; or each relay should disable the other one, perhaps by having each coil wired between the controlled center rail and the isolated rail of the same path.

It would not be necessary to cut into the crossing if both tracks are powered from the same source. The control and controlled sections could easily be isolated from the crossing, which would remain powered all the time.

Even with separate power for the two tracks, you could use one of the relays to decide which source powers the crossing and still avoid surgery. A form-C (SPDT) relay could do the entire job.

[Big_Boy_4005]

Right Bob, the insulated control sections don't need to be long, 10" is just fine there.

I took a little time to whip up a diagram. All of my relays are 3PDT's. Most of the time I don't need the extra contacts, but on this application the third set is for the signals.

The general idea is that the ground for each relay is run through a set of contacts on the opposing relay, thus allowing only one to be activated at a time. Even if the second train arrives, it cannot energize it's path until the first train is clear of the crossing.

It looks very complicated, but it really isn't if you study it. The drawings enlarge and clear up when you click on them.



Here is a diagram of the layout where I first used this. I didn't just have it control one crossing. It controlled a cluster of 4. It was very cool, because it allowed the trains traveling parallel to pass, while the perpendicular movements were stopped.

This layout ran 4 trains with automated passing sidings, in addition to "malfunction junction".[swg]