Mike,
In the U.S. all pressurized vessels are regulated by the D.O.T. and are subject to periodic testing. The actual term is "hydrolastic expansion test". The test is designed to show if the container is sound enough to contain operating pressure in the designed manner.
As has been stated, water has almost NO elasticity so once the pressurized vessel is completely filled and air pressure is applied the test pressure is reached almost immediately. What the actual test pressure is varries with each container, function and location!
There are two types of hydrotests; high and low pressure. The type that is used to qualify locomotive boilers is the low pressure test and is the one that we're interested in. (High pressure hydrotests are used for CO2 cylinders, Scuba tanks, oxygen bottles, etc... and is a completely different kind of test.)
Once the test pressure is reached (let's say for example 1 1/2 times rated operating pressure of 160 lbs.) which in this case would be 240 lbs. the pressurized vessel must maintain that exact pressure for a specific period of time. If it does, great! It passes. If it doesn't and there are no leaks anywhere then what the lowering pressure tells us is that the vessel is expanding due to the overloaded pressure which is an indication of a weakening of the walls of the vessel and imminent failure. The vessel then fails the test.
It is much better to find out that a pressurized vessel will fail during a hydrotest than in actual operation due to the water (remember about no elasticity?) Well, if there is a catastrophic rupture the pressure will immediately drop to zero and all that will happen is that some water gets sprayed!
D.O.T. is very particular about these tests and requires complete documentation. That's one of the reasons that steam railroads have to jump through so many hoops. It's all in the interests of safety.