A monoploy isn't based upon whether there is an alternative, e.g. train or bus, it's based upon whether one organisation has complete control of a particular market, this market being a rail route between A and B.
Can other rail companies enter this market? No. It's a monopoly.
If I lived in a town were the only 3 supermarkets were tesco's, then they would have a monopoly on supermarkets in that town, although I would always have the alternative of travelling to another town to go to Morrisons. This wouldn't mean Tesco's didn't have a monopoly. After all, I would always have the alternative of not eating.