Little Hans just got a model railroad kit from his grandfather for Christmas.

His grandpa explained to him, that when he was young, he was a model railroad enthusiast, and encouraged his grandchild to not despair if he cannot put together everything correctly the first time.

There is no shame in it, as his first attempt at model railroading was also unsuccessful: he just modeled the station of his home town, Friedrichsfeld .

But when he wanted to start the Mannheim-Heidelberg express train, his little train just wouldn't start. He checked every cable, every connection, none of them were broken, but he still couldn't start his little train.

The question: did the grandfather use an AC or DC power supply for his model railroad?

I've only chosen Germany as a location because both AC and DC was commonly used. This puzzle has a logical explanation, I didn't want it to make it into an assumption like "in this country AC (or DC) was very uncommon".

• While the name of the boy and the occasion he got the present (it could have been his birthday) are not important, every other information in the text can be important in some way or another.
– vsz
Commented Dec 28, 2014 at 17:55
• Do we need a "electrical-engineering" tag? Commented Dec 29, 2014 at 10:50

He used DC power. Looking up Friedrichsfeld, it looks like this:

DC-trains usually do not have the third rail that is common with AC-systems and instead simply use one rail for power and the other as ground. If you try to build a triangle with a system like that...

...you get a nice, little short. This is a common beginner's mistake, the same thing would have happened with a loop (although those are much rarer in real railway systems).

• Indeed. This "AC = third rail" is so ingrained into public consciousness, that even today, completely digital models with a third rail are commonly referred to as "AC models" and without a third rail as "DC models", even though they no longer have an analog power supply, neither DC nor AC.
– vsz
Commented Dec 29, 2014 at 9:36
• This is why the grandfather was important: back then there was no digital control, and models with a third rail had AC motors in them. The purpose for the distinction was that DC motors have a polarity-dependent direction, so it doesn't matter in which orientation you set them up, you can use an absolute direction: they use two rails, and a positive voltage on the right rail means forward. In case of AC, you can't control the direction via the polarity, so a high-voltage "direction switch impulse" is required. As polarity doesn't matter, the two outer rails are on the same potential.
– vsz
Commented Dec 29, 2014 at 9:43
• American Flyer trains (S gauge) were powered by AC delivered via the left and right rails; they had the same problems as DC trains. I also believe at least one company made DC trains with a center conductor. Commented Dec 30, 2014 at 0:24
• @supercat : then I'm glad that I specified Germany. In Europe the "AC = 3 rails / DC = 2 rails" distinction is so strong, that many people, including some manufacturers call even modern, digital models AC or DC, depending on the number of tracks. They sometimes even identify with AC a digital protocol used by a company which in the past made mostly AC trains. I'm still thinking how to improve the question to make it more fair, and to eliminate John Zwinck's solution without spoiling this solution too much.
– vsz
Commented Dec 30, 2014 at 9:18
• @vsz: I like John Zwinck's solution, though it interacts interestingly with the AC/DC distinction. American Flyer trains switch direction when current is applied/removed (in the sequence forward/stall/reverse/stall, so that if a train hits a dead spot while in motion it will stall rather than reversing direction immediately). Commented Dec 30, 2014 at 18:17

The grandfather used AC power.

Many model trains are powered via the two rails they ride on (rather than a "third rail" as in many metro trains and also Lionel model trains).

The express train had two locomotives. Had they used DC power, their orientation would not matter: facing the same way they would drive together, and facing opposite directions, the "backward" facing one would also reverse its motion (because its contact with the rails was backward too), making the whole thing move forward.

But with AC power, the direction of a (single-phase) motor does not depend on how it is wired externally. So the two electric locomotives facing opposite directions simply played tug of war.

• A nice answer, and I'm sorry that I didn't consider this in my question. I've rarely seen non-freight trains with multiple locomotives, and when the grandfather was a kid, most locomotives were steam-driven, where it's unlikely to not notice a wrong orientation. I should have been more specific with the "wouldn't start" bit, to specify that the train did nothing, the wheels didn't turn, etc. It's also highly unlikely that ones first attempt would involve a multiple unit train. Still, +1 for the lateral thinking. You also provided a lot of hints for a single-locomotive solution.
– vsz
Commented Dec 29, 2014 at 8:54
• @vsz: Freight locomotives are often deliberately staged back-to-back, so that they operate as a single twice-as-powerful locomotive which can switch its cab-first direction without having to turn around. Commented Dec 30, 2014 at 18:36