Since WT upgrades can come with the reindeer, the second level 1 train showed up earlier than before. You may not be able to use it now, but you may be able to use it later. If you buy the reindeer, you may also end up with a few extra upgrades for it as well. As we've seen with recent events, we'll likely get the chance at more upgrades and cars next year and sooner or later, they will end up in the AD.
You may not have room for it now, but your city will grow. You can let it sit in inventory or you can sell it to the AD. Just keep in mind anything you sell to the AD will be much harder to get again later on. For instance, let's say you end up with an extra level 1 WT and 2 WT upgrades in inventory. that gives you a level 3 WT sitting in inventory, leaving only 6 more WT upgrades needed for a second level 9.
Let's say sometime between now and next year you decide to sell them to the AD. Next year when the WT shows up again, you'll need to chase down and acquire 3 extra WT items if you want a second train. Having to chase those 3 extra items means you miss out on what? Who knows?
