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.
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.
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.
I don't know; that would depend on what stage they were at with their normal workload and routine. Check, and respond accordingly.
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.
Either sell it at a reduced price, or donate it to your local foodbank, homeless charity or women's refuge.
With a bit of imagination, motivation and good leadership skills, I see no reason why it couldn't be that simple.