The traditional model railroad power and control has transitioned from simple hardwired loops to advanced networking in the past decades, bringing many benefits and a few drawbacks.

DCC is a real step forward, but what comes next?

Model railroading is an activity that seems not to need battery power or wireless control of the locomotive. After all, the model layout is in a confined, fixed location, and there is plenty of available AC power to provide the modest 20V/2A maximum needed by a typical model locomotive via a line-sourced power supply unit. Even better, the tracks of the layout also can literally function as power rails and deliver that power to the locomotive as load. They are a free and available power-transmission subsystem.

As noted in Part one, there are two ways to control locomotive speed and direction. In the older system called cab control, you adjust/reverse the voltage applied through the track being sent to the engine but have to deal with power blocks and switch supplies to the appropriate bock as the locomotive travels along.

In the newer and very popular digital command control (DCC) system, you instruct the locomotive via digital control codes, which are sent using the rail itself as both the communications medium as well as a constant-voltage power source. A decoder IC in the locomotive reads the code and controls the DC voltage applied to the motor while track voltage and polarity stay constant. Either way, it all seems so simple, so straightforward – the availability of basic supply power and the tracks as power-delivery rails seem to be a near-perfect fit to the application.

The reality is that it is not such a good fit despite its apparent simplicity. It turns out that the obvious method of using the tracks as power (and as digital command rails, with DCC) has lots of issues and problems:

• Power comes to the motor via the wheels and is then “picked off” by small metal fingers that rest against the wheels. It’s mechanically tricky and electrically subject to intermittent due to dirt or oil at both the track-wheel contact and the wheel-pickoff contact.
• Every wheelset on the locomotive and all cars needs to be insulated, so the metal wheel-axle-wheel combination does not short out the track (many of the cars use plastic wheels, partially for this reason).
• Whenever there is a gap in the track, such as at a rail turnout (switch), the conduction and connectivity are momentarily lost. For this reason, many locomotives get power from more than one axle, which increases reliability but adds to the complexity of pickup arrangement. You can also buy and add special modules to the locomotive, which has a supercapacitor (and a charging circuit) to provide carry-over power through the gap. Of course, these add to cost, plus it’s tough to find a place on-board for these.
• Plus, there’s one big problem for which there is no good answer: the reverse loop. Any track configuration that permits a train to change its direction without simply backing up needs a special electrical circuit to prevent a short circuit. Unless it is a simple circular or point-to-point track topology, a track will loop back onto itself at some point in the layout configuration (Figure 1).

The result is that the positive rail gets connected to the negative rail, yielding a short circuit across the two (thankfully, the term “ground rail” is never used here by modelers – nor should it be). To avoid this problem, it’s back again to cutting track gaps in the rails at appropriate points to create isolatable blocks. Once the locomotive is in a gapped block, the polarity of the track leading to the block is manually reversed using a double-pole, double-throw (DPDT) switch (or via an automatic circuit you can buy for about $30) so when the locomotives come out of the loop, the feeding track polarities agree. It’s a real nuisance to have to deal with and manage the power switching at these reverse loops, even if done automatically.

Part One and Part Two

The next and final part looks at a very different way of powering the locomotive, eliminating all the issues associated with using tracks as power rails.

EE World Related Content

• Electric locomotives and catenary power systems – Part 1: basic functions
• Electric locomotives and catenary power systems – Part 2: power needs
• Electric locomotives and catenary power systems – Part 3: power delivery
• Electric locomotives and catenary power systems – Part 4: maintenance and corona
• Westinghouse and the fail-safe train air brake, Part 1: The problem
• Westinghouse and the fail-safe train air brake, Part 2: The solution
• Westinghouse and the fail-safe train air brake, Part 3: Electric brake control
• Westinghouse and the fail-safe train air brake, Part 4: Post-Westinghouse
• Sorry, but it’s “Goodbye, Caboose” – EoT devices have made you obsolete, Part 1
• Sorry, but it’s “Goodbye, Caboose” – EoT devices have made you obsolete, Part 2
• Electrified Locomotives, Tunnels, and the Pennsylvania Railroad: Astonishing engineering but a partially sad ending, Part 1: The challenge
• Electrified Locomotives, Tunnels, and the Pennsylvania Railroad, Part 2: The tunnels
• Electrified Locomotives, Tunnels, and the Pennsylvania Railroad, Part 3: The station

Other References

1. Model Railroader, “How to wire a layout for two-train operation”
2. National Model Railroaders Association (NMRA), “Beginners guide to Command Control and DCC“
3. Southern Digital, “How DCC Works”
4. DCC Wiki, “DCC Tutorial — Basic System”
5. CVPUSA, “The AirWire900™ Battery Powered Wireless DCC Control System”
6. Wikipedia, “Connections” (TV series)