Before I can answer I have a couple of questions for you.  Are you sure you found a rat and that it was not a mouse?  Was the rat a male or female? 

My parents recently had an infestation in their home.  It took weeks to catch the rat, but it was only one rat.  They were lucky, because had it been a female they could have had a hugh infestation.  Typically rats seek out other rats so they may reproduce.  So there may be more where your rat came from.  They typically find holes on the outside of the house where they can make there way in.  It would be a good idea to look around outside at ground level.  You may find an entry point.

You have to be very careful with rats because they like to chew.  They will chew wiring, drywall, insulation and many other materials.  There have be many instances where rats have caused fires by chewing wires.  They also carry disease so make sure you clean areas where rats have been.