Yes, there is a mounting hole in the center of the bridge rectifier shown.The one shown in the picture is a typical package. Whether or not you use a heat sink depends upon your usage. If you run long trains with big locomotives it's wise to have the bridge rectifier mounted on an aluminum heat sink. In my case, I ran many of the smaller LGB locomotives for many years and never had any overheating in my dual 5 amp power pack.
That has changed now, however with my acquisition of the USA Trains F3 A-B set (Boston & Maine) The two units together with their four motors pull three amps, and will cause some heating of components, not just the rectifiers. So my power pack is now pretty much obsolescent. That being the case, I'm in the middle of building a ten amp dual power pack. That will have a bridge rectifier on each side and each will be mounted on an aluminum heat sink. This could be something as simple as a piece of aluminum bar, say 1/4" X 2". I will also have a 120 V fan that runs all of the time that either side of the pack is being used. That should take care of the problem.
The suggestions above about using an LED of green or red to detect polarity are good, although there has to be some track power on in order to fire up the LED. Typcally a red or green will start at something like 2 volts.
Good luck!
