I would recommend--if at all practical--sticking to one battery voltage. Without putting too fine a point on it, "idiot-proof" should be your mantra. One battery voltage means one charger (or multiple identical chargers). This way you're not playing "match the loco to the charger" and picking up the pieces
when someone guesses wrong. Given the time frame (c. 1900), you're going to be using steamers, so in all likelihood 14.8 v. Li-Ion packs will prove quite adequate. (They have for all of the locos I've worked on.)
For a set-up like yours, here are my suggestions:
1) I prefer to make it so that I can remove the batteries for charging. That way if a battery pack goes flat, I can just swap it out and keep operating that same locomotive with a fresh pack. A standard connector for all batteries--one that can only be plugged in one way. 2-pin connectors like the ones Aristo and USA Trains use in their locomotives are available from allelectronics.com. I use them and they work well. There are other connectors out there as well.
2) As best as possible, standardize how and where the batteries are changed. Most of my steam locomotives have removable coal loads that allow access to the battery below. If the coal load is not removable, the entire tender shell is. (I don't recommend removable shells for a public display run by volunteers unless you really enjoy repairing tender frames when someone forgets which is which--not that I've ever done that. Never...

)
2a) If you go with on-board, non-removable batteries, do your best to make sure the charge jacks and power switches are in consistent locations. Also, try to make the connections between loco and tender as simple as possible, since folks will be taking trains off the track whenever the batteries go flat. If you're going the non-removable route, fit the highest capacity battery possible in the locomotive to maximize run time. A 4800 - 5200 mAh battery should give you 6 hours run time, maybe up to 8.
3) Consider the possibility of using a power car instead of a fully on-board installation. I'm typically not a fan of power cars, but I do a lot of prototypical switching where always having a box car connected to the locomotive gets in the way. In a public display such as this, it makes sense on a few fronts.
First, there's the ease of installation. With everything in the power car just feeding motor voltage forward to the locomotive, you don't need to do much (if any) modification to the locomotive. The only thing you would have to do to each locomotive would be to isolate the track pick-ups from the wheels. In some cases, the manufacturers have made this simple with a "track/battery" switch. Tie into the board where the battery leads connect, throw the switch to "battery" and you're off and running. In other locos, it's not difficult to isolate the track pick-ups from the wheels themselves (cut or remove the brushes), and then wire the pig-tail to the wires those pick-ups used to connect to.
Second, it saves on cost if you're going to have a small fleet of locomotives, but only a few tracks on which to run them. You'd need only one power car/control receiver for each car, as opposed to one for each locomotive. Box cars and baggage cars make great candidates for power cars. The Li-Ion batteries can be slid through the side doors very easily.
4) With multiple operators, you're going to want multiple transmitters. From my experience with the public display at the Colorado RR Museum, "simple" is good. While some systems allow you to operate many trains from one transmitter, resist the temptation to do so. These trains will be operated by total technophobes, and all they'd need to do is accidentally press one wrong button on the transmitter and they're all of a sudden not controlling anything with no idea what they did or how to regain control. We use track power at the museum, but each track has its own--well identified--controller. (And even then, some have managed to get confused.) Even if a transmitter can be programmed to run a gazillion locos, just program it to run one.
Later,
K