The only train would be the class 710 although you could add the class 315 and 317 for a bit of nostalgia. This route would probably be the most easy London overground route to make alongside the Romford to upminister route.
thanks mate, I’ve edited my reply now and yes I do know that the whole of the London overground is quite long so I’ve chosen my favourite route.
Give more detail on different rolling stock, route length etc. Read some other suggestions here and format them like that.