The 20 minutes rule for passengers is (in addition to competition by private cars) one of the few incentives to expand to faster trains and a more efficient network comprising in-city transportation. It really rewards players to build a well thought train network with customers wanting to travel farther and thus generate more revenues (or at least this is my assumption, in reality I don’t know how the rule really works in selecting which locations passengers want to go).
For freight it could be a very different story. I agree that relaxing this rule would be beneficial to gameplay, but there are two more important topics: clever industry placement (alas it is completely random) and slow industry production rampup and maximum levels (waiting for demand to produce, little demand and production increments and little demand and production overall limits) really limit the need for long freight trains… I would like production to increase at least 5 times it does now (and of course reducing prices) … in that way trains would be more competitive to trucks and long heavy freight trains would become a necessity and feel different than passenger trains (adjusting the weight of freight cars could also be a possibility).