Rail is pretty much dead and buried in the Traveller universe with its cheap, compact, reliable, and safe fusion power and ready access to contra-grav with the same properties as the fusion reactors. If you can move a massive starship effortlessly using contra-grav, there's no way a "fixed schedule" train is going to be competitive given the large infrastructure costs of setting of trains, at least from a technological point of view.
However, there are non-technological factors that might make a culture use train-like modes of transportation.
For instance, what if a world wants to keep its skies clear of flying cars, starships, and so on? This is entirely plausible:
* People in cities might consider all these flying cars, starships, and so on to be an eyesore.
* There may be very real safety concerns about all these flying cars/trucks/starships zipping around their skies. Particularly in these Traveller-style universes, it appears all these vehicles are human piloted and basically like a modern automobile. They're hazardous, totally dependent on human pilots who may be distracted, asleep at the wheel, drunk, or simply have bad judgment. Given how many times a year you see stories in the local news about cars swerving off the road and doing whatever, think about how many times worse this would be with these vehicles that are a cross between a speed of an airplane, the landing properties of a helicopter, and the ease of use of a modern automobile. It'd be a nightmare. I'd be more surprised if worlds let people just zip around as they please. Remote-controlled vehicles or AI piloted vehicles might still have safety concerns as well.
* There may be concerns about the costs of all these flying vehicles running over cats/dogs/birds or whatever their local equivalents are.
* Speed. The noise involved in all these sonic booms of aircraft might be an issue on High-Population worlds. While most contra-grav vehicles can't go this fast, there's no reason why they can't with the proper design. These supersonic aircraft flying around everywhere, again, would be a real problem. If a world is high-tech enough to have perfected vacuum trains or possibly even some derivative of supercavitation, you might get a kind of "tube train" (essentially a subway). The same technologies that make anti-gravity fusion wonder vehicles possible would make it reasonably cheap to make such tunnels/tubes - fusion boring could make tunnel-construction very cheap (of course it would, this is totally idealized, just like Traveller fusion power and contra-grav - of course there's no issues with loose rock, earth quakes, or anything like that, it's not like contra-grav emits dangerous radiation or hazardous electromagnetic fields even though it seems likely they would) and this space is either kept airless or filled with some high-tech gosh-wow fluid. When you have like a Mach 10 train running between cities, and even doing Mach 1 within cities, cheaply powered by onboard fusion power, you'd have trains again - of a sort.
* To control the movement of people; approved mass transit makes it easier to monitor and control the movement of people. Now, given the rabid freedom-mongers that seem to infest the RPG community, this does not need to be a goose-stepping oppressive government. For instance, I've always wondered, given the nature of Traveller, what really prevents some criminal from showing up at a planet, hopping out of the downport with some Air/Raft, going somewhere and shooting at the locals with TL15 fusion rifle until it runs out of power, zipping back to the downport (so the police can't chase him there because of extraterritoriality concerns) and jumping out of the system in a real hurry before the Imperials and locals can work things out? Once the guy leaves the world in his starship, he's free. He can just cruise around space being a shooting-spree criminal and nobody can do a thing about it. Sure the world might hire bounty hunters (like the players) or something to track the person down, and it might give your players some rosy righteous feeling of "having brought someone to justice" but "justice" isn't going to bring the dead back and people on these worlds are going to start asking "is there any way to stop this kind of thing from happening?" Another example: A Scout ship seems very aerodynamic - what really prevents someone from approaching all nice and civilized until he's near a big city or something, and "buzzing the tower" going like Mach 3 at nearly street level (because he's modified his Scoutship to be capable of 6G maneuver) then zipping out of the system before anyone really has a good response? More prosaically, all these grav vehicles zipping around could make things like customs checkpoints really problematic, particularly if the world itself is balkanized; smuggling, kidnapping, and so on would become much easier; police would have to track things in three dimensions. Grav vehicles are fast and very maneuverable and that means police response has to speed up to prevent people from getting away. That can get very expensive to have such measures in place. (The typical RPG gamer response is "why not just shoot them down?" ... seriously. If you lived there, do you REALLY want some futuristic equivalent of Patriot missiles or something ringing every city and InterstellarArms Sunfusion X-ray Laser Satellites orbiting over your city? Let's talk about That Oppressive Feeling.)
I'm sure there's other reasons, but those are the ones that immediately come to mind.
If there are laws against it, the world might have fixed-route transport - restricted to roads or corridors. Basically like trains. Trains might be faster because of speed limits on grav vehicles. Restrictions might fix grav vehicles to set paths in the sky like the "roads" of flying cars in like the Star Wars prequels or movies and deviating from those paths might require expensive permits or "off-route" time charged by the distance or something. Such local conditions might make fixed-route transport economically competitive.