A thought occured to me... why not compare the cost of the Lightning Rail to that of the time-honored way of travelling long distances in D&D: the
teleport spell?
Teleport, cast at the basic 9th level of ability, transports the caster and three others up to 900 miles. The normal fee for spellcasting is spell level * caster level * 10 gp, or in the case of
teleport, 5*9*10 = 450 gp, multiplied by two because you're also paying for the caster's return trip - so 900 gp. You can split this cost three ways, so we get 300 gp for instant travel of 900 miles - that's 33 coppers per mile, or 2/3 of the cost for a trip along the Lightning Rail. Now, let's compare some other things about the two modes of travel:
Lightning Rail pros:
- You can travel shorter distances, and/or by yourself and still get your money's worth.
- It's probably easier to book transport on the LR than it is finding a 9th level wizard who knows teleport (and house Oriens charge 10 gp per mile for teleporting, which is a lot more expensive than the formula above)
- The LR can transport cargo more effectively.
Lightning Rail cons:
- Limited to travelling along the actual rail.
- Have to wait until a train leaves (how often that happens isn't specified in the book).
- You "only" travel ~600 miles per day (assuming the train takes breaks at stops, and only travels effectively 20 hours per day).
Teleport pros:
- The time taken for actual travelling is instant.
- You can go anywhere the caster is willing to take you.
Teleport cons:
- You need to find a 9th level wizard (or 10th level sorcerer) willing to cast the spell for you.
- You will probably need to wait until next day so he can prepare the spells (which may or may not be longer than the time until the next rail leaves).
- Slight risk for accidents in transit, depending on the caster's knowledge of the destination.
- You always have to pay for the full 900 miles of travel, even if you're only travelling a relatively short distance.
Still, with these things in mind, I think it would be reasonable to price travel along the Lightning Rail at, say, 2 sp per mile for first-class accomodations and 5 cp per mile for second-class. The Rail and
teleport both have different advantages, but I think
teleporting wins out in the end, but not by that much.