1.The train runs 60 miles an hour. 2.The train runs 60 miles per hour. What is the differen between the two sentences.
They have the same denotation, as you'll find in a dictionary, but "a/an" sounds a bit colloquial to me. Although I often hear and say "a/an", I would only write (and normally see written) "per", except in very informal circumstances.
It's a bit different.
Per = pro(German) = for --> for each hour of running a train will cover a distance of 60km
con = against
an hour = in an hour --> the train can cover a distance of 60km inside the timespan of an hour
Both are saying the same essentially. The one just focusses on if you have one hour only, how much distance can you cover in! that hour, while the first one talks about how much time or hours you would need in order for! you to travel a certain distance.