autotrain

Among other fun things with the model railroad, is the ability to automatically control the trains. This has been possible for a very long time by switching turnouts and signals; the signal cutting the current to the rail when it says "stop". Digital operation opens up more possibilities, or rather, makes the train control easier and better. Here the whole concept including traditional operation is handled.

Input devices

There are three major ways for the train to indicate its position: the circuit track, the contact track, and reed contacts.

The circuit track is a special track, available in all M, K and C track fashion, half a track in length and in straight, radius 1 and radius 2 type. It has a little switch mounted in the middle operated by the trains pickup shoe(s). When operated it closes (grounds) one of two connections to ground (normally, the C track could be close connection to other connections), depending on the direction of the trains travel. It's a reliable way to control trains conventionally, with the benefits of sensing the direction. The major disadvantage is that multiple pickup shoes (light in cars) make things very complicated.

The contact track utilizes the fact the Märklin is 3R. In the contact track one of the rails is isolated, and a cable is connected to this rail. When a train enters the section, the trains axles closes the ground connection to this cable. The device exists in several forms; M has special tracks for this, with C (and K?) track it's easy to build such a track yourself. All track types also have this kind of ready tracks for railroad crossing operation, which may be used for any purpose. The switching is continuous as long as a trains is on the section, and direction independent. Might be an advantage, if you want to indicate position, but is normally a disadvantage. With end switch off contacts in turnouts and signals (as with C and newer K track turnouts) it's OK to use these tracks, and they are also suitable for digital operation, but not so for M-track turnouts.

The most professional device is perhaps the reed contact. It consists of a small metallic tongue in a glass tube which has two connections. When a magnetic field is present, the tongue makes contact with the other connection. It's as simple as that. The tube is placed at the track, between the rails or beside. Permanent magnets are placed in the loco (or in cars if wanted). When the train passes the contact is closed. The closure is momentaneous, and if the magnets are placed on the side (and the reed contact) the closure will be direction dependent. If only placed in locos, you will not get the problem of the circuit track, that you get several contacts for a train with car light. Our experience with reed contacts is limited.

Train control devices Turnouts and signals can always be used to control the trains, directly in traditional switching, and via accessory decoders in digital operation. In addition to this, in digital operation you may control the trains directly, provided you use a computer, and keep track of the trains position. Turnouts and signals (mostly C and K track) may have end shutoff contacts, meaning that if fed with continuous switching power, they shutoff automatically when they have switched. This means that contact tracks may be used as well as circuit tracks, unless you need the direction dependency. Logic devices Before the digital systems there were one way to extend the logics: to use universal relays like the 7045 and 7245. One of our favorites was to use the extra switching possibility of the signals; when catenary isn't used, you can use the catenary train control connections of the signal for your own purpose. To save money that is, the same function is achieved with an extra relay. Another possibility is to let one signal switch on and off the switching power of another device. The signal must must be a light signal - semaphores don't have the distant signal light connection. And the controlled device should have separate feed for any lamps. All 72xx have this, also C and K turnouts.

Traditionally the switching was done directly; when the track closed the circuit, current went through the coil and the device switched.

With digital there are more steps. The input devices are connected to a s88, which reports changes in the state of the device to a computer, or a memory unit. The computer and the memory are capable of sending switching commands to the accessory decoder, and speed commands to the locos. The possibilities for logics are unlimited.

Beside these standard solutions, there are electronic equipment from other suppliers, to get trains hold at a station for a time, to go back and forth automatically, to control hidden yards, to gently slow down and accelerate, etc.

Digital equipment (6088) The s88 has 16 input connections. Each contact may have one of two states: 0=not grounded, 1=grounded. More exactly, it indicates 0 if the connection is open (not connected) or has a continuous potential not going below + 2.5 V (roughly). When it is connected to a lower potential, that that, it indicates 1. So, it may be used with the circuit tracks etc, but also a connection to "red" will do, and to AC with common ground with the digital circuit. Examples of usage is to sense if the bridge of a turntable closes the contact to a specific track, etc. I have used it in a special way; I wanted to know if a signal was red or green, and I was using catenary, so no switching capabilities left. However, the s88 was connected to the red of the track (actually, the catenary). When the signal was green, it was closed, indicated 1. This was the case also when a train was present (the track was "grounded" through the motor), but that didn't matter. I was able to catch when the signal was set to red (at that moment I knew there was no train present), and that was all I needed.

The s88 may be connected to an interface for further transmission to a computer, or to a memory unit. The computer has unlimited possibilities, the memory is preprogrammed device, still having lots of more switching possibilities.

The memory can take up to 3 s88 modules, each s88 controlling 8 of the 24 "programs" in the memory. Connection 1-8 of the s88 when closed, activates the corresponding programs, and connection 9-16 releases them. Each program may contain 20 switching operation, or time delays, or nesting to other programs.

Common mistakes Do you want to make your own mistakes? Feel free to do so, and have fun. Or read further.

When controlling conventionally, one mistake would be when one want to control signal A from two different tracks, 1 and 2, and signal B only from track 1. The straightforward connection would cause also signal B to switch at track 2. One solution could be to have two circuit tracks at 1, one for each signal, another to have one relay which decides which track controls signal A. A third is to use a signal with "orange" in addition to "red" and "green"; orange could be
used at one track and green at the other (if switching to "run" was wanted).

When mixing AC and digital, I connected a signal and a s88 input to the same circuit track. This caused the s88 to always be on; it was grounded through the coil of the signal. The solution here was to connect the s88 to the signals catenary output, and monitor if the signal was red or green!

It has been said again, but it's very difficult to have cars with sliders while using circuit tracks.

When switching digitally, think about having a separate circuit for the accessories. The problem could be that when several trains are running, you get a voltage drop, and the power left is not enough to switch some accessories, maybe causing a crash.