i recently started an N scale model set. the shop that i went to for the loco and the cars that i have all had the same couplers. i went in to the same shop 3 weeks later and they had went to a train expo to sell their n scale stuff and possibly accuire new modeling items. i had noticed they purchased a pack of random new cars with the different couplers, they all had knuckle couplers compaired to all the cars and loco that i had purchased the 3 weeks prior... am i able to get the knuckle couplers and replace the old ones or do i need to get new trains?