On Saturday evening the 2d inst. as the last train of cars from Lowell was approaching Boston two of the hindermost cars accidentally parted from the train in Medford, about five miles distant. The occurrence was not discovered, however, until after the conductor had supped in the city and returned to the depot, when he was astonished to find that two of his cars, containing some 100 passengers each were "among the missing!" He instantly dispatched a locomotive on the return track, and the lost cars, with their population, were brought into the city after a detention of about an hour and a half, by this both amusing and vexatious oversight.