There is an article or editorial in a major 3 rail train magazine, latest issue, that cautions users to not leave a transformer unplugged but connected to the same section of track as an energized transformer. This is a real shock hazard as the snippet points out.
However, there is another very important issue that can is dangerous when multiple transformers are energized and hooked up to the same section of a track or layout. It's called phasing! There probably have been many threads discussing this in the past, but if you are new to trains, and use multiple transformers to power the same section of track to obtain enough amperage for your trains, you MUST properly phase the transformers!
To test 2 transformers for proper phasing, have them disconnected from the layout. Have no wires on the secondary's(s). Plug them both in to your AC mains. Push each throttle on each transformer to maximum position. Take an AC voltmeter, and measure from U post on the first one to the U post on the second one. You should measure zero volts. (You should have both handles at maximum when doing this!) If you do measure zero volts, take a drop of paint or some other way to mark how to plug the mains plug into your receptacle. Mark the same side of each plug. Whenever you plug them in, be sure to keep the colored dots to the same side of the AC receptacle. This is PROPER phasing1
If you do measure a voltage between U posts, your primary windings and AC plug are NOT connected in phase. To fix this, simply reverse one of the plugs, and test again. Remember to have the handles at full throttle. This time you should measure zero volts. If so, paint the drop on the same side of the plugs as above.
A "gross" test, if you don't own a voltmeter is to connect a wire to one of the U posts. Again, insure both handles are at full throttle position. Quickly touch the wire to the other transformer's U post. If it sparks, your transformers are not in phase. Reverse one of the plugs, and repeat this test. Should not be a spark, this time. This is NOT the recommended way to test for phasing, but it will work.
Safety is so often overlooked by many when dealing with multiple transformers on one layout. Take a little time, and be safe! Ask questions of this board if you are ever unsure of any situation involving transformers, and AC line voltage. There is an issue that can occur with very old transformers called leakage, but this text won't get into that. This type of problem is best handled by a qualified repair shop. Before you use a really old transformer, have it checked at a LHS for leakage.
Be safe, and have fun running your trains!