Dennis, Polar Express is, according to Jerry Calabrese, the best selling single train set in the entire history of Lionel. Whether you'd find the set after Christmas at a reduced price is in part a dealer and/or wholesaler decision. And in part also influenced by how many were made for this particular retail season, and the quantity remaining after Christmas in distributor/Lionel stock.
Usually it is the lesser popular sets that end up as blow outs or reduced prices. The NASCAR 2-4-2 starter sets and the Berkshire Major League Baseball sets come to mind. They were decent sets, just not that popular with buyers initially until the blow outs.
Right now Lionel direct has the Pet Shop and Dinosaur starter sets for $60, which is certainly a well discounted price. The other similar sets with real railroad names (Santa Fe, Pennsylvania) are nowhere near that price.