Ok, some things to think about.
Let's place strategic supplies of bottled water at main stations along the route. How much are you going to need 1 or 2 pallets, or more?
Keep an eye on how fast your stocks are dwindling, and with the resources available, respond as best you can.
Not necessarily; start with the ones where you usually supply replenished stocks of food and drink to trains, but be ready where necessary to provide stocks of a particular item - bottled water, in this case - at one or two other locations in addition to this, where this is viable and practical.
Bear in mind that if there's 700 people on a train then this equates to around 60 cases, which is around a pallet. How many trains worth of water do you keep at your strategic location?
Where are you going to store it?
I don't know, that would depend on how in-demand a particular item - in this case, bottled water - might be, and what kind of storage space you have available, and the capacity of said storage space. Where possible, keep an eye on demand, and respond accordingly. Store it where you normally store it, and make use of any appropriate alternative/overflow storage space you might have available, either at your normal station or, where possible, also one or two alternative ones at which trains will also be stopping, for the temporary period of time required.
How are you going to get it from wherever it's stored onto the train? The chances are it'll be in a station building so you'll need manual handling equipment, it might need to have go across the footbridge.
Who is going to move it? Have you trained them to move bulk amounts of water (yes it's only water but if you need to get 60 cases of water to a failed train the person doing it needs to be properly trained and have the right equipment)
The same way you normally do, with the job carried out by the people who normally do, plus other people willing and able to give them a hand if required - ideally, these would be immediate departmental colleagues doing some overtime (an extra hour or two at the start/end of a shift, or extra staff on the same shift, perhaps) - but only if you have sufficiently good industrial relations and team spirit, and can do this within the bounds of relevant legislation.
Required training can be incorporated into your normal, routine on-the-job training, and/or made part of your job induction process. Refreshed at regular intervals. All part of what I said about anticipating unfortunate turns of events, and being ready for them. Plenty of other industries do this.
What job is the staff member moving it not going to do while they move the water?
I don't know; that would depend on what stage they were at with their normal workload and routine. Check, and respond accordingly.
What happens if the train breaks down away from the station? Let's say it breaks down in a remote area?
Let's just get some water from the nearest strategic station.
Who's going to move it? Is there a lorry driver and vehicle available? Do you expect the railway to pay a retainer to have this service available 24/7?
How long is it going to take them to get to the station with the water.
They've then got to load up. Does the vehicle have the right equipment to do this (thinking tail lifts, fork lift truck etc.
How long will it take the driver to get to where the train is stranded?
Who unloads the water at the other end. Is the right equipment available....
This has not been part of the discussion so far, and I have never suggested that anything like this should be attempted. My point was, is, and is likely to remain being that supplies of water be made available at stations to be delivered to trains once said trains reached those stations, and to do the best you can within the resources you have available plus any extra resources upon which you might be able to call, to look after your passengers as best you can, with as cheery a disposition as you can muster. And be as visible as you possibly can whilst doing it, as part of a genuine effort to look after your passengers and give as positive an impression of the railway in general, and your organisation within it in particular, as you can to them.
The water will have a best before date. If it's not used what are you going to do with it? Who's going to move it etc. etc.
Either sell it at a reduced price, or donate it to your local foodbank, homeless charity or women's refuge.
It sounds really simple but it's nowhere near as simple as it sounds...
With a bit of imagination, motivation and good leadership skills, I see no reason why it couldn't be that simple.