I´d vote for "capacitación"
I absolutely agree with you, Laura. At least in the office jargon in México, "capacitación" would be the usual word in this sentence, but I would change it a bit:
workers training: trabajadores en capacitación o bajo capacitación,
meaning that the workers are getting through a training program. I think training is functioning here as a verb, that is to say, "what the workers are doing". If the activity were "una práctica", the sentence would be "workers practicing" instead of "training".
Saludos