>>1903072>Unsure what they mean by heavy railnot necessarily a bigger grade but bigger trains in general
think about those trains that have 2 levels
basically what you were thinking about but no luxury
the s bahn trains are 99% never compatible with the infrasture of theu bahn trains
Hamburg has an interesting example
they have a number of u bahn lines (blue, red, yellow, turqouise)
and an s bahn netwrok (green, purple, maroon) which splits on the north side of the central station into a "inner city loop" that rejoins north of altona at diebsteich
such an s bahn inner city loop could work for
>>1901173 