It could be the train is converted to use 'Motor-Man' doesn't that provide acceleration/deceleration simulation on trains with the chips fitted? On those you also need somewhere to put the RC batteries as I recall, although I've never dabbled with it myself.
As for a Li-Po battery, these are typically used in small drones and model planes where you need a very small/light but relatively high capacity battery. They are readily available and are basically a rechargeable lithium polymer battery cell. I think you do need some special sort of charger though to make suere you don't overload them.
The thing that caught my eye was the 'MP3 player' module. I don't know if he has taken this from a commercial MP3 player (which can be bought very cheaply these days) or if it is just an off the shelf chip/module you can buy. It is probably that which requires the Li-Po battery as they are usually higher than 1.5v I believe.
Think I need to do a bit of research to confirm all this
As for a Li-Po battery, these are typically used in small drones and model planes where you need a very small/light but relatively high capacity battery. They are readily available and are basically a rechargeable lithium polymer battery cell. I think you do need some special sort of charger though to make suere you don't overload them.
The thing that caught my eye was the 'MP3 player' module. I don't know if he has taken this from a commercial MP3 player (which can be bought very cheaply these days) or if it is just an off the shelf chip/module you can buy. It is probably that which requires the Li-Po battery as they are usually higher than 1.5v I believe.
Think I need to do a bit of research to confirm all this
Happily collecting things all my life... 
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