Not sure what you mean by the Costa Rican "Northern" RR. Costa rica has an eastern (Atlantic) line from the capital of San Jose to Limón. The other section is the western line (use to be electrified) from San Jose to Puntarenas on the Pacific. Some of the Pacific section is likely still operating. The Atlantic (Caribbean) line does not operate and its San Jose station is now a museum (open by appointment only) with a nice steamer and cars on display. The gauge of the aforementioned is 3.5 feet.
There are also NG lines in the south, running into Panama. I believe they are 3 footers. Got to go down there on my next tripand see what's left of those sugar plantation lines. If something is there I'll send photos of steam equipment to J. Hefner.
Don't know of anything in northern Costa Rica??? Does anyone else have thoughts about that?