Some background thoughts on real life trains
My understanding of how a lightning rail train works is that it's levitating, rather than riding on rails or anything like that. That said, here's some reasons why real life trains can be difficult to move in:
- Obstacles like seating, handhold bars (in a subway car), people, luggage, etc. These would apply depending on what type of car you're in. If you're in the cheap seats and it's crowded, it's going to be slow going.
- The cars are connected to each other by some method, but meant to be generally isolated. They have a connection to move between them, but even if the train is going straight it's not open terrain. If the train is turning things will be moving around somewhat and make it even slower going.
- The big issue with trains is that tracks are sometimes not perfectly level. There are slight bumps and jostles, which make waking harder compared to a surface that holds still. I don't think this would apply to a levitating train like the lightning rail. Consider an airplane: walking around on a plane that's at cruising altitude is really easy, as the floor isn't moving around on you. In turbulence or during takeoff/landing it's much harder to hold your balance (and not recommended for safety reasons!). So I'd actually say that moving inside the lightning rail is a bit easier than moving inside a real train.
On the roof? Well, you've got all kinds of difficulties there:
- The terrain itself isn't really meant for walking on. It might be painted, have vents, be designed for aerodynamics (on high speed trains), or any consideration other than someone walking around on.
- At speed, you're going to have a whole lot of wind resistance on your body when you're standing up. Aside from making it hard to walk into it, it'll be pushing you against you while stationary. If the train is going fast enough it would also impact your vision, as it's harder to see clearly with an 80km/h wind blowing in your face.
- You probably don't have any kind of designed walkway between cars on the roof, so you're jumping. If you're going forward and the train is going at high speed, you're jumping into significant wind resistance.
Game Mechanics
All that said - your idea for inside the train was to apply a balance check to move full speed. That may or may not make sense, depending on why you think they'd have a hard time moving. Based on my above thoughts about a lightning rail being a smoother ride than a train, I wouldn't personally use that. I would apply the difficult terrain rules:
Difficult terrain hampers movement. Each square of difficult terrain
counts as 2 squares of movement. (Each diagonal move into a difficult
terrain square counts as 3 squares.) You can’t run or charge across
difficult terrain.
If you occupy squares with different kinds of terrain, you can move
only as fast as the most difficult terrain you occupy will allow.
Flying and incorporeal creatures are not hampered by difficult
terrain.
My reasoning is that they still have to deal with obstacles like the seating, baggage, and possibly other passengers if the train is occupied. I don't see the ride being bumpy enough inside the train to actually knock someone over very easily, but a balance check doesn't let you get around that it's a cramped space with stuff in the way.
On the top of the train is another story. A Balance check seems like the most appropriate skill check in 3.5e to move in those conditions. There is the risk of falling off to consider in that case, which is a risk that doesn't really exist inside the train. There's also more room with less stuff in the way, so if you can move safely you wouldn't have to worry about passengers, seats, or luggage to be in the way.
One added wrinkle is that Balance normally allows you to move at half-speed, unless you take a -5 penalty to your roll. Another consideration is that a DC 14 check at level 2 will be rather difficult for members of the party without a good DEX score, as not many classes will have many ranks in balance by level 2. Many classes won't have the skill points to have any, and with a DEX of 12 they would need to roll a 13 on the dice, which means they'll fail a majority of the time. Balance also has a risk of falling if you fail by 5 or more, so setting the DC too high could result in fatalities if they go up there, roll badly, and fall off at high speed.
Exactly how you want to do that and what DCs you want to use will depend on things like how fast the train is going and how high you want the falling off risk to be, so it's hard to give you some precise values. To give you some ideas, here is how I would handle it if I were running the game.
Assuming the train is moving and occupied with a modern passenger train load (people have seats, nobody is required to stand, as opposed to a packed subway at rush hour):
- Movement inside the train: No check required, difficult terrain rules apply (that is, you move at half speed)
- Movement above the train: DC 10 Balance check to move at half-speed. That would allow them to optionally try for a 15 to move at full speed, that they would fail to move on a 6-9, and risk falling on a 5. Anybody with no DEX bonus and no ranks might want to think twice about trying it, but dexterous party members would be fairly safe.
Now if you want it to be riskier or more difficult for them, raise the DC. 14 would give them the risk of falling on a 9, which would be VERY dangerous for someone like a Cleric.
Although movement in 5e is not governed by squares, they are probably a helpful model to this particular exercise. Everything in the universe is effectively measured in 5' increments and using squares to model this will give us an effective answer to this question Let's posit a 50' wall
x
x
x
x
x
x
x
x
x
x
Now let's look at the veritcal space next to the wall
yx
yx
yx
yx
yx
yx
yx
yx
yx
yx
Finally we need to have the flat part on top of the wall, and the space above that
yyyyyyyyy
yxxxxxxxxx
yx
yx
yx
yx
yx
yx
yx
yx
yx
Now, supposing our hero starts at the first y (marked a below), and runs up, he runs out of movement at the 10th y (marked b below).
yyyyyyyyy
bxxxxxxxxx
yx
yx
yx
yx
yx
yx
yx
yx
ax
If you start at a, you might be able to climb up onto the top of the wall. However, if you have a 45 foot wall you are safe. If you have to move to a, a dash would be required to not fall (as you don't have the ability to stay vertical).
The problem here is that if the wall extends at all into the square above the one marked b, then you're in trouble, you have to move into the square above it as diagonal movement is not possible.
This will be both at the discretion of the map makers (if the building extends a bit above the square you're probably not going to move diagonally) and also your DM (he may allow a bit of wiggle room here). Consult your DM before you attempt any 50' wall climbs.
45' wall climbs (and lower) are safe though.
Best Answer
This works, the spell water walk defines snow as a liquid for mechanical purposes.
Water walk says:
Here snow is described as a liquid for mechanical purposes. I argue that this demonstrates that for mechanical purposes, the rules are respecting snow as a liquid.
Thus, I conclude that Unarmored Movement does work on snow, as snow is elsewhere described as a liquid in a mechanical context.
In the comments Mark Wells raised an objection worthy of consideration. He writes,
My argument relies on an important assumption, based on the "Spells only do what they say they do" principle: a spell only changes the mechanical nature of something if it says it does. Else, we assume the nature of the thing is used consistently within and without the spell's description.
An example of this can be seen in the spell storm of vengeance. The effect during rounds 5-10 of storm of vengeance state:
Under normal circumstances, environmental phenomena such as wind and rain do not require a caster to make concentration checks, unless the DM determines they should. Storm of vengeance specifies a change in the nature of this mechanic when it says:
The spell description specifies that we treat the nature of wind and rain differently for the purpose of the spell.
Therefore, since water walk does not specify that we are respecting snow (or any of the other terrains listed) as mechanically unique for the purposes of the spell, we conclude that the rules are consistent in their treatment of snow both within and without the spell description of water walk.