I have heard that Nicaragua used to have an operating train system, but at some point in the past it was dismantled and discontinued. Are there currently any trains that regularly operate in Nicaragua? If so, are there any that a visitor could ride on?
Sorry, no trains.
I am not sure about freight trains, but there are no passengers trains in Nicaragua anymore since 2001.
Wikipedia has a page about the history of rail transport in Nicaragua
In fact there is only one passenger train in the whole of central America and that's a tourist train along the Panama canal.
Further north in Mexico there are lots of freight trains but very few passenger trains.