Some of the answers depend on where you are modelling, specifically what species of trees are common and how big they typically get. In my part of the world (British Columbia) the original track construction resulted in complete deforestation of large areas on either side of the track for grading, timber requirements, fire and etc., so no trees close to the tracks would be older than the tracks. However trees grow big and fast, particularly on the coast but less so in the interior, so even a young tree (by BC standards, where trees can live for 100’s of years) can be pretty darn big. Also, in the days when lineside communications lines were critical for operations, I suspect the railways would have been pretty careful to keep the tree line at least one tree height away from the pole line so that a single fallen tree wouldn’t tie up the line. Perhaps more important on mainlines than secondary/industrial… I also wonder if fire protection was a consideration.
Another consideration would be what tree species are most likely to grow back first in a disturbed area. In my neck of the woods, that “pioneer species” would be Red Alder and other deciduous species, with conifer species coming later, often by a couple of decades. So the consideration for that ‘between the track’ area would include the date of construction or last significant upgrade or revision to the track (last disturbance), track maintenance standards standards and geography (tree species and growth rate, which might also include just how hospitipal that particular location is for tree growth and health, and what species. (Sorry, I’m in the ‘urban forestry’ business.)
On my layout I’ve planted dense forests of conifer trees behind the tracks, but these trees are smaller than what might be expected in the wild because I’ve also got the tree line closer to the track than is proto (not enough room to have the treeline 100 feet from the track and have a representative forest, and to ‘force the perspective’ of having the treeline farther from the track than it actually is. Large portions of my tracks run parallel to the Kicking Horse River, so the question of how to deal with trees in front of the tracks is reduced (but not eliminated), I also intend to keep that treeline distance more prototypical in front of the tracks than behind to further reduce the foreground forest requirements. Where appropriate I will plant just enough trees to suggest ‘forest’, and these trees will closer to ‘full size’, but there aren’t really a lot of areas where large forest areas will be needed in front of the track. I’ll be looking for that well composed Rocky Mountain tourist photo look, with just enough foreground trees to frame the image.
