I've discovered OpenTTD this year and am having a good time playing it

Currently I'm playing using the xUSSR set, but I do struggle with the trains whose passenger wagons have Cargo Age setting as "Instant". They seem to barely cover their running costs, and the later versions do not cover they acquisition/replacement costs, despite running almost full. I become bankruptcy after some years when I try to start employing them. If I only use slow age wagons it is fine, on the other hand.
So my questions are: What is the mechanic of different cargo ages? What are instant cargo age passenger trains used for?
I play on standard size maps and use the JGR Patch, shall this play a role.
I already thank you for an answer and for this wonderful game
