I was searching around and came across this manufacturer of battery and diesel locomotives (plus they make wagons). I thought they would make a really interesting basis for a modern day narrow gauge layout:
https://www.locopro.eu/en/diesel-locomotives (https://www.locopro.eu/en/diesel-locomotives)
https://www.locopro.eu/en/battery-locomotives (https://www.locopro.eu/en/battery-locomotives)
They also publish the dimensions on the site so you would not even have to guesstimate 8)
There are other manufactures as well as these that make similar locos.