"E-boilers"
That is just the advantage of working with what I call an "E-boiler", in other words it is an electronic simulation and you don't have to live (or die) with your mistakes. The lack of results for a low water incident (insufficient water to cool the crown-sheet [top of the firebox]) is on the same level as the lack of a detailed collision system in Trainz, it appears that Auran didn't feel it was necessary. As the majority of the users of this program probably have no real life experience with a boiler of this size it is probably wise of Auran to let them "get away with" having a low water incident. This allows people with no experience with steam systems to enjoy steam locomotives in the game without forcing them to learn how to operate a boiler.
I suppose it would be possible to create a rule to immediately terminate your driving session if the locomotive you are "on" has met the requirements for a low water incident. A low water incident does not always result in a crown-sheet explosion; however the safety systems involved can fail, alternatively there may not be sufficient water on top of the crown-sheet to douse the fire in the event the fusible plugs melt. In any event the locomotive will be rendered useless until the damage is repaired, if that is possible. If a low water incident results in a crown-sheet explosion the crew will not be around to help with subsequent tasks.
If you are really interested in learning more about boilers it is certainly possible to do so as there are many resources available to do so. Learning about how a steam locomotive works is some what harder as it is a more specialized field of study, in addition it is not an area of great demand. However it is possible to find materials about the subject.