I know a guy who is a plastics mold designer - not for train stuff
but for stuff that is similar in size (slightly larger). It is my
understanding that the molds used for plastics are considerably
different from those used for resin kits.
The majority of the reason why this is true is due to way the
plastic molding machine 'runs' during a production run. Those
machines 'assemble and take apart and assemble and take apart'
the mold at a very high rate. The liquid/heated plastic is 'shot'
(forced under pressure) into them during the brief time they are
assembled - then the whole thing is taken apart (think "several
sliding parts of metal" that are pulled apart in a sequence), the
part is ejected into a bin, the mold is re-assembled ... ad
A resin mold is a two part mold and the resin part sits in it
until it 'cures'. Most of the resin parts/kits we use are
probably not even run thru a molding machine like the plastic
parts/kits ... it is likely to be a 'hand process'. This is
one of the reasons why flash is so different between the two