There's actually a connection between canals and railroads, at least topographically. Within a few hundred yards of my house here in the Denver area there is a canal called the High Line Canal. It meanders some 75 miles from where the South Platte River exits the mountains on the SW side of Denver all the way here to Aurora, where it finally disappears in the plains a bit NE of where I live. The settlers built the canal by hand to carry water for irrigation about a hundred years ago. It's still in use. I cannot imagine how they built it, but it follows a line that is a gentle drop from the mountains all the way to here, which is only about 25 miles as the crow flies.
But how does this equate to railroads? My five year old is a train freak, and I take him on train rides occasionally. Two years ago we took a narrow guage line from Chama NM to Antonito CO. The last third of it was across the plains. That train followed the same sort of line that the High Line Canal follows. Basically the path of least steepness. It was easier and cheaper to do this than to run in a straight line, because they would have had to fill in dips and cut through hills. Trains have a physical limit on how steep a slope they can climb.
