Student or Learner
The long length of the train will compensate for it's low frequency. The train can carry/hold more passengers, even though the frequency the train services is low. The short length train cannot carry too many passenger, therefore the frequency of the train services is higher in order to serve/attend to all the passenger.
You usually give your threads more helpful titles than this, scott. The first sentence of your post would have made a great title.
Incidentally, there are three sentences, so Are these sentences right? would have been better. And Please advise would have been correctly spelt.
Note that the possessive pronoun its has no apostrophe. It's is the contraction of it is or it has.
'Longer trains will compensate for the infrequency of the service.'