Probably a matter of taste, but that very situation is part of what, IMO, Eureka did better than Diadem. By restricting movement and incorporating a leveling system, you really felt like you conquer Eureka bit by bit. The more you delve into Eureka, the more active you can be, from a solo wanderer to a full backup on the train, to a valuable DPS asset, to a train leader or train main tank. You also have to learn how each species react, wether they aggro you on sight, sound, low HP or magic if you want to navigate the zone at earlier levels..And it's godsend they didn't allow flying in Eureka, because the minute you fly, everything stops being a threat.
I didn't have that many opportunities to participate in Diadem trains, but I didn't see that much collaboration discussed when compared to Eureka. In Eureka, death bears a penalty, so, players are inclined to be helpful towards dead people, sometimes really far of the map, probably because they expect others to do the same for them.
For me, Eureka has only one major flaw (But it could gain a lot of various improvement), it's the lack of level sync that makes partying with friends very tricky. Also, most people board the FATE train even though the FATE Xp penalty you suffer when underleveled could make it a very inneficient way to gain Xp depending on your level. Almost every instance I was in, there were enough low levels to make at least two parties. The challenge bonuses might help to switch away from this paradigm a little in the future.
