It's just the same as "ловив". More like "while he was waiting for a train...". I whould say "continous" (in english) or "imperfect" (in russian). Middle of the process.
Difference is in strong binding to the past in case of "ловив". This process strictly was in the past. "ловя" can be bound to any time - from past to future, it derives it from main verb.