Electronics and DCC

 I am completing the construction of a single-track drop-leaf bridge at the entrance into my train room, and am wondering if there might be a way to wire it to be foolproof, to prevent trains from dropping off the edge of the world.

 I know I can just use the bridge itself as a sort of SPST switch, completing a circuit to allow power to its rails and some portion of the approaches only when it is up and in place.  However, this just gives an "emergency stop" with voltage dropped all the way to zero.  I'm using analog not DCC, I should clarify.

 So I was wondering, might there be a way to have another section of approach, which always has its voltage set at the average between the active main and the bridge?  Thus, when the bridge is up, the main might be powered at 8 volts, the bridge at 8 volts, and thus the approach is 8 volts too.  When the bridge is dropped, though, the average between 8 volts on the main and 0 volts on the bridge is 4 volts on the approach. This would slow the train down, prior to stopping it when it reaches the 0 volt block.

Electrical stuff is my one major weakness in this hobby, so I would appreciate any help I can get.  If anybody has some other effective solution, please let me know!

Now that I think about it, one issue that could rear its ugly head is a train that is backing up, sending the unpowered caboose first down into the abyss...

(if anyone wants to see my track plan, go to http://wpandp.com/HomeLayout.html - it is "Capon Bridge" that I am talking about)

 You just need more contacts - either use more than one switch, or have the switch trigger a multi-pole relay. One set of contacts controls the bridge and the immediately adjacent tracks - either power  is on, or it's completely off. The next set of contacts powers the next block out from those dead ones - either power is connected as normal, or it goes through a resistor - it needs to be a hefty one, 1 amp at 12 volts is 12 watts, so at least a 20 watt resistor. Value to be determined by experimentation. The problem with this is, of the train is already goign slow when it hits the 'slow down' section it's just goign to stop anyway. Unless you run very long and heavy trains with live loads and/or pushers on the back, and unless you have a habit of runnign them at warp speed, hitting a suddenly dead section isn't goign to be an issue.

 Make the power off sections as long as your longest train and even if you backed in, the loco would hit the dead spot before the caboose dropped off the cliff. Unless you run trains so long that this isn't practical.

                                         --Randy

 The main dangers I am facing are these.  On one approach, the train climbs a 3/4 turn helix which happens to be set at my minimum radius (15" in N-scale).  I am hoping to treat a portion of this curve as a curved trestle.  If the train stops too abruptly, I am worried about a derailment on this trestle, which would send a car crashing to the floor.  Obviously, I could forgo the trestle, but I'd rather forgo the derailment potential.

The other end of the bridge has a turnout leading into a siding, and that siding feeds a major mine.  There could be all kinds of switching moves and/or meets going on here.  Perhaps I could just depower the entire town if the bridge is down.

As for making approaches as long as a regular train, this might be feasible but it will definitely mean cutting power to the entire town on that side.  The partial helix on the one side is intended to be about the length of a train, such that the train completely exits one town before entering another.  I could make this whole helix serve as the approach block.

 Ugg, just lost a long reply. ANyway, the only way I can see doing the slow down is a device called a negative temperature coefficient resistor. What these fancy words mean is that it's a device that has low resistence when cold, but it increases when it warms up. You'd have to do some experimenting to find the proper value to use. What would happen is that with the bridge down, the NTC resistor would be wired in to the track power. When the loco hit that section, the resistor would pass whataver pwoer the train was drawing, and heat up. As it heated up, the voltage to the rails would drop and the train would slow gradually. Most would probably stop before the got past the slowdown section, but a short section ot totally dead track could be a last resort safety measure. Better to derail some cars then have a loco hit the floor.

                        --Randy