Why do you suppose he experienced these strong cravings at a particular subway stop? What principles of learning explain these cravings?
The fundamentals of a type of associative learning are working in this process of getting ill again.
In classical conditioning, a previously neutral stimulus (conditioned response) is repeatedly paired with an unconditioned stimulus. Next, the response that was evoked by the unconditioned stimulus (unconditioned response) is now elicited by the conditioned stimulus (conditioned response).
In this case, the unconditioned stimulus (US) is the heroin substance, and the unconditioned response is the intense urge to consume it. After pairing this US with the location of a particular underground railway station (this could occur by the person intaking this substance outside this location regularly before), the person developed an association between them. So, when the person reached this stop, it elicited the learned response, which is the intense urge to consume the substance.
The person may have abused this addictive substance at the underground railway station or around it. Therefore, he could have developed an association between these stimuli, that is, the intoxicating substance, and the location where he abused it.
Principles of classical conditioning are operating in this scenario. The underground railway station became a conditioned stimulus and evoked the response of an intense urge to consume the addictive substance.