So this is what it says in MyTrainz under Redeem Trainz code:
TANE Platinum Edition
Step 1: Install Trainz A New Era if you have not already.
Step 2: Enter your username and password in the game Launcher (Trainz Settings > Internet tab).
Step 3: Click "File > Download Purchased items.
Step 4: Navigate to the Routes Menu and the Asset Download window will appear.
Step 5: Select which content to download or click Download All.
I may have misunderstood, but it reads like there was some kind of code to download Platinum Edition if you already had TANE (of which I already purchased before the free version came out). Once I read the rest of it this morning, it probably has to do with obtaining some other promo code to download addition routes or something. IDK. but it was just kind of confusing. Will most likely purchase Platinum Edition anyways, so as to fix the messed up SP4 error with some routes/content shown as purchased when in fact they aren't.