GTA V featured Los Santos' metro, but it was largely background. RDR2 actually had working trains. The case is unclear for GTA 6.

What's plausible

A Vice City metro - scenery-based or functional? That depends on Rockstar. A long-distance Florida-style train (real Florida does have Brightline) - further into speculation territory.

Evidence of trains in the trailers is scarce. We'll update this page if more emerges.