To begin, probably best to compare the lightning rail to maglev trains. The fact that they hover probably indicates that there won’t be too many bumps (though I’ve never ridden a maglev train so I can’t say for sure). Speed is probably a bit more constant than it is on a real train, too, which makes keeping your footing easier.
Inside the train, DC 10 Balance checks even seem high. The problem is, if you are balancing and don’t have 5 ranks of Balance, you are flat-footed, which doesn’t seem appropriate here. I’d probably just do something like: each round the DM rolls a 1d4; on a 1, everyone must make a DC 10 Reflex save against the jolt. Failure forces you to make a DC 10 Balance check or fall prone. Considering that I ride considerly-less-smooth subway trains every day and have never been knocked off my feet by a jolt, even these numbers might be high, but then people in combat are a bit distracted.
Other considerations are likely a large amount of obstructions, and any acceleration or deceleration that the train is undergoing. Trains tend to try avoid any sudden accelerations or decelerations, but while those are going on you are effectively on a sloped surface: from the perspective of someone in (or on) the train, walking towards the front of a decelerating train (or towards the rear of an accelerating train) feels like walking downhill, while walking towards the rear of an decelerating train (or towards the front of an accelerating train) feels like walking uphill. The DMG has rules for fighting on slopes, so those should probably be applied.
Outside the train, the balance problem becomes more severe. The “floor” in such cases is not designed to be walked on, may be quite smooth, and may be rounded, sloping off towards the edges. Now serious Balance checks are appropriate. At level 2, the maximum rank in Balance is 5, Dexterity scores are, at best, 20 (for a +5), and taking feats or anything like that improves Balance checks is a rather unusual choice (and anyone who did so deserves to be “off the curve” so to speak). On the flip side, it wouldn’t be too hard to have a −1 or −2 to the check, so that’s the range you’re looking at, −2 to +10.
Given that a +10 requires a fair amount of investment and implies that the player should be good at this, that should probably be an almost-automatic success. Meanwhile, the guy with −2 shouldn’t be completely barred from the fight, though he’s going to regret that −2 at least for now. As such, a DC in the 13-15 range seems appropriate. So your guess of DC 14 is pretty much spot on.
I will comment, however, as someone who spends a lot of time on a train, though, and wouldn’t put my modifier on Balance above +2 or +3 or so (certainly never trained in it aside from playing as a child, not a particularly dexterous person, but I’ve noticed my sense of balance seems decent compared to those around me who didn’t do gymnastics or anything like that), I’m not sure that I buy that I could stand on a moving train with 40%-50% reliability. So if you want to go for verisimilitude, the stunt may be beyond your players. But frankly, that’s kind of boring, and totally not what Eberron is about.
Finally, while you’re outside on a moving train, you effectively have a wind in your face (assuming you’re facing forward) equal to the train’s speed (which, for the Lightning Rail, should be quite significant). The DMG and SRD have rules for handling high winds.