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".