This is exactly how the trains in Prague metro are taken out from the depot. The extension cord is connected to the end car closer to the shed's/workshop's door and the train is shunted to the front of the shed/workshop. When the front bogie of the end car touches the 3rd rail, the extension cord is disconnected and the train rolls out by it's own. The remaining cars get powered, as the train moves out of the shed/workshop and their bogies touch the 3rd rail. When the train is returning to depot and it's entering the shed/workshop. The extension cord is connected to the end car closer to the shed's/workshop's door and the train shunts inside the shed/workshop. After the train shunts in, the extension cord is then either disconnected entirely or reconnected to the end car closer to shed's/workshop's door.Sjoerd wrote: ↑28 Nov 2022, 11:18 An alternative way of doing it might be to have a long extension cord which you couple to the train in the workshop, long enough to supply power for the train to drive out, and then uncouple it again once the train comes in contact with the powerrails outside. But I could not find any information relating to this, so I guess the Unimog is the simpeler solution.
Yeah, the train can also shunt inside into shed/workshop without getting connected to the extension cord, but with extension cord connected, the driver can move the train slightly, if he/she stops incorrectly inside the shed/workshop and has to move a bit to correct it.
The way, how the train was connected to an extension cord varied over time. The early version of the extension cord had a small insulated contact, which was connected to the bogie next to the 3rd rail shoe. This was meant to be compatible with trains from Russia (Ezh3, 81-717), which were used that time. Although this design was simple and easy to use. It was slightly dangerous. If the depot worker connected the extension cord, turned it on and then touched the 3rd rail shoe. He/She will get a very painful 750 V DC zappie.
As the time progressed the extension cord was redesigned and now it has a plug, which looks similar to European 3 phase plug. Also the extension cord isn't connected directly to bogie anymore. The trains, used nowadays in Prague (81-717M, Siemens M1) have a socket hidden below the masks on the end cars, where the plug is connected.
Also the extension cord isn't just laying on the ground. It's hanging from the ceiling connected to a small contact trolley. This allows train to move inside the shed/workshop without worrying about the extension cord getting caught, tangled or twisted somewhere.
Also, for safety reasons, the extension cord has a switch placed somewhere nearby. This allows the depot workers to switch off the extension cord, when it's not used, to avoid unwanted and painful zappies from it. Just imagine, you forget to switch the extension cord off and you touch the contacts with 750 V DC in it. Ouchie! If you can't imagine, how it's painful to get zapped, go watch Mehdi Sadaghdar alias "Electroboom" on Youtube. He sometimes zaps himself during his tests and his reaction says it all about the pain from getting zapped.