Railroad trestles serve the following purposes:
* Act as an overpass for trains. No-intersection crossings, allowing long RR-freeways that don't get interrupted when they pass through congested areas.
* Overpass is safer for human-crossings, too!
* Act as a mid- and end-game use for wood! (Currently, all our wood goes into storage that feeds 2-3 old-school furnaces, which is kind of lame.)
* Challenge mode: Trestles are also impacted by cliff explosives. Of course, they're super-susceptible to damage from a variety of sources, as anyone whose ever watched an old Western knows.
![Smile :)](./images/smilies/icon_e_smile.gif)
There's more, as you can imagine, but that's the basic bullet points.
Every time we start a new factory, by the time we get to somewhere between lasers & nuclear power we're sure wishing we had RR trestles!
(The group I play with starts a new factory every few months, either when there's a new [big] update or if someone wants to try some new challenge, whether achievement-oriented or not. "No lasers" made for a weird game (we always play with aliens on); our current is "no artillery", meaning map exploration has to be done "by hand".)