The value of R1 depends on the AC transformer that you use.
For a 9V AC transformer, it would be 0Ω (i.e. no resistor).
For a 12V AC transformer, I believe I calculated something around 150Ω.
It is just to bring down the maximum track voltage.
It's a basic voltage divider circuit:

You can use an online calculator to get a value. However, the actual track voltage will be two diode drops (V
BE = 1.4V) lower because of the darlington.
https://ohmslawcalculator.com/voltage-divider-calculatorOr just put a meter on the output and try some values in the 0Ω to 200Ω range trying the limit the track voltage down close to 12V. Definitively, keeping it below 14V.
Note:
If you don't use R1 and you're using a 12VAC transformer, the track voltage will be about 15.5V. Which IMHO is a little too high for a 12V motor.
Note 2:
I'm forgetting about all the diode drops at the front end. So, the voltage will be a little lower. Just leave R1 out and check the output voltage. Then adjust R1 accordingly.