Exjpansion and contraction due to temperature means the metal pieces will expand and contract against each other. In a garage, basement, or attic without full-time climate control, these changes can be relatively large. This, with other influences like vibration from running trains, will gradually loosen rail joiners against the rail. This will increase the likelihood that the current will not be transmitted from one section of rail to the next, and the train will stall.
As I mentioned, though, this is only one of the influences that tends to interrupt current between sections of rail. If you ballast your track, you have that much additional dust, glue, and dirt trying to work its way into the joint. Ordinary non-ballast related dust and dirt as also a factor, as is corrosion. The result is definitely that performance can deteriorate on a layout as time goes on.
The solution for this part of reliability is to have multiple feeders for electrfical sections and to solder rail joints whenever possible. As I said above, a layout may work for a while without doing this, but the joints will deteriorate over time if you don't.
A Kadee track gauge or alligator clips from Radio Shack will work as heat sinks, too, by the way. But a melted tie now and then isn't a tragedy. You can cut out the damaged one and insert another cut from a spare section of track if you have to.
