Everybody remembers these black marks, they were typical for the steam era. But I am modelling the diesel era, 1965, when nobody spoke (yet) about pollution and the exhaust (on top of the locos) emitted black smoke (sometimes worse than the steam locos ! ). So, should I reproduce those stains too ? This topic is neither a joke nor a "Troll", I saw this mark on the picture of the concrete tunnel portal by DeBenLLC ... but there is a steam engine running out !
Last Edit: Jun 6, 2019 14:44:28 GMT -5 by alberich
I'm not sure if there is an easy answer. It would depend on how you envision the railroad and the landscape. You'll generally have some mineral staining across the top of the portal from water running down the slope above. How much depends on how you think the rainfall would be. There might even be streaks of green algae if it's wet enough.
For the exhaust stains it would depend on how busy your rail line is and as a previously mentioned, the strain the locomotive is taking.