Builds are authorised through the resources allocated to your MyTrainz account. Buying a product in the store allocates that resource to your account, or signing up for a membership provides different resources.
If you want to check out TRS22 the cheapest way would be to upgrade from Silver to Gold for a month, which gives you access to S20 as well.
If you want to purchase TRS22 make sure you use your discount code (SILVERDISCOUNT) for 10% off.