It sounds better and the rationale can be that the X45-line, X21-line, X10-line etc etc, collectively are the Xlines. Ta-Da!
Calling them 'lines' as well just makes it seem a little more... special, maybe, rather than ordinary bus 'routes'. And lines make it feel more like its part of a network, (the Xlines network). Maybe thats the thinking already, but I certainly believe my NSA suggestion would sound better, even if I do say so myself.
