All aboard! Next stop… limbo. Forever.
Much like water, trains show up a bunch of times in Inception. The first time we see a train, we're in Japan. Cobb and his gang are riding on a train as they try to extract information from Saito. You've may have forgotten about that train—it isn't really symbolic, per se, but it does give us a bit of foreshadowing.
Cobb says he's "getting off at Kyoto" and when Arthur says that "It's not like he's going to check every compartment" Cobb answers, "Well, I don't like trains." This isn't just a weird tick of Cobb's or some bizarre train-phobia. As we later learn, Cobb actually has some pretty good reasons for not liking trains.
Trains are a big ol' deal because Cobb and Mal committed suicide in Limbo by lying down in front of a train. Why did they pick a train, though? Cobb and Mal were essentially gods in their own Limbo-verse and could have chosen any way time die (being smothered by kittens sounds good to us, as does overdosing on Ben & Jerry's.)
It probably has to do with a cutesie couple saying they have that goes like this:
"You're waiting for a train; a train that will take you far away. You know where you hope this train will take you, but you don't know for sure. But it doesn't matter, because you'll be together."
D'awww, that's actually pretty sweet.
This cutesie saying matters a lot to Cobb. Not only does he have a special "train level" in his memory-dream that Ariadne passes on her way to the basement, but he also brings a train into Yusuf's dream right through the center of the street. What's important about trains is that they are essentially unstoppable (although Denzel begs to differ) and have only one course they can take: one path.
There is a certain security in this that Cobb and Mal cling to and that Cobb (in his subconscious as a portrayed by projection Mal) still clings to because he is uncertain of everything else: if he has killed his wife, if he is dreaming, if he'll ever see his kids again.
Thinking of life as a journey and that journey as a train keeps him from going crazy (or at least too crazy). It keeps him hopeful.