ASOS is "action short of a strike" - it means that training keeps being put back as a result of industrial action.
This is only half the excuse, if an employee is supposed to receive X days training, familiarisation, safety update days per year etc as part of their normal work then they won't be affected.
ASOS (action short of a strike) is only not doing extra duties, voluntary tasks etc, doesn't affect any tasks normally booked to be done.
However some operators have sort of reallocated staff to more essential duties, instead of routine training. Strictly the employee participating in ASOS should turn up at whatever their roster said at time of calling ASOS, ie if they were scheduled to go to a training centre that's what they do, not accept a late change to driving instead.
In practice, bit of a grey area, and many employees don't understand the nuances of ASOS, instead mistakenly think it means doing core duties, instead of duties originally scheduled. So if someone says can you drive today instead of doing training, the often forget they are doing ASOS and agree to the change.
Really ASOS will only affect training during overtime, not scheduled training, so when you hear training is postponed, it's only bit that was unscheduled or overtime that gets postponed, not all of it.