You can use either Basic Speed Control (Vmin, Vmid, Vmax) to limit the speed, or else  a Speed Table, which allows a non-linear curve (also limited at the bottom and top ends).  Both methods allow you to keep the top speed as low as you like, however they are applicable on a locomotive-by-locomotive basis.  It just occurred to me that you might be looking for a way to limit the top speed on _any_ engine dropped at random onto your track.  The answer to that one is beyond my ken.

David

All the N Scale setting does is lower the voltage to the track. This won't hurt the motors as they are receiving less top end voltage.  A  better way is to program the CV's in each decoder. You can program acceleration,deceleration,  starting speed, mid speed and high speed. This gives better control of the locos.  You didn't say what brand decoders you have and if they're motor decoders or sound decoders.   Joe

The best way to limit speed, is to set the maximum top end speed with the decoder CVs. This can be a pain, especially when you have a lot of locomotives you want to set. I would use JMRI to easily set the CVs. It gives you a graphical user interface (GUI), will allow you to quickly set and test the new settings to see if they work the way you expect, and will save the settings for each loco so that if they should ever get wiped, you can easily restore them.

The software is free, and should run on just about any computer that will run java.