Hi Greg,
To the best of my knowledge the yard in the vicinity of the main depot at Merida was 3-rail, dual gauge (3' and standard gauge). However, there were numerous smaller gauge RRs in the area ranging from 18" to 24" gauge. I understand these were used primarily for local, "push car" service. I only know of their existence because they were convered in Trains magazine some years back. It is entirely possible the three gauges converged at some point in the Merida area, but I have no specific knowledge of this. Perhaps someone else does.
On a completely separate, but tangentally related note, I have your "Cuba: Stronghold of Steam" and the Railfilms Indian Steam Volume 3 tape, which I'm pretty sure my wife bought from you. Thanks for making this material available. I look forward to future installments.
Best Regards,
Glenn