You can currently try out all Trainz DLCs for free until January 15th if you own either TRS19 SP4 or the beta of TRS22.
To do this, log on to mytrainz.com and scroll down to "Offer 2", there you have to press the "Claim" button and then you can download all the DLCs you are interested in in the Trainz DLC Manager and try them out by January 15th.
On January 15th, everyone who took part will receive a 75% discount code for the Trainz Store, which you can use to buy DLCs you liked and keep them permanently.
The code can probably also be used for first class tickets.