To the best of my knowledge, as a former Rock Island Railroad employee, no specific standard that requires that there be sanitary facilities any specified distance apart. That said, back in the day, except for shelters and relay boxes which were intended for the protection of equipment and supplies, and were not intended to be occupied much of the time, contained sanitary facilities. Yard jobs were generally given a task by the yard master at the yard office (where there were facilities), went and performed the task, and then came back to the yard office for the next assignment, so they were regularly in the vicinity of facilities then. In large yards, such as hump yards, where there would be people stationed at some distance from the yard office, people in remote locations generally had shelters or shanties where they could wait, and these also usually included facilities. Many cabooses also had a built in rest room for the rear end crew to use.
Water tanks probably did not have separate outhouses, but water tanks were most likely locations where there were other facilities, such as an operator's office, which would have contained facilities. But a train and engine crew who stopped a train only for the purpose of making a visit to the sanitary facilities would have been subject to discipline. In the days of steam, engine crews operated over a particular subdivision, and knew exactly where they could "take care of business" without having an unexpected audience.
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