"under training" is correct and usable in written English.
You can use it to refer to a period of time during which someone is learning something new or developing a skill, for example, "My new employee is under training to learn the accounting software".
The new national army is still under training and his militia is weak.
I had more than 100,000 troops under training or busy with operations.
These are believed to be raw recruits who have been under training.
Yet too often individuality is crushed under training schedules and obscured by the demands of "teamship" and the unspoken requirement not to bite the hand that feeds.
A more general sense is defined by the O.E.D. as "engaged in or connected with active (military) operations, as distinct from being under training, in reserve, etc".
In July 1918, impelled by dreams of martial glory and by despair at a broken love affair, Faulkner joined the British Royal Air Force (RAF) as a cadet pilot under training in Canada, although the November 1918 armistice intervened before he could finish ground school, let alone fly or reach Europe.
Jeremy Clarkson faced further censure on Saturday after describing people who killed themselves by jumping under trains as "selfish".
Jeremy Clarkson has faced a further backlash over his outspoken comments after he described people who kill themselves by jumping under trains as "selfish".
The latter constraint ensures the avoidance of HMM under-training occurrences.
The athlete must walk the tightrope between over- and under-training.
This choice is based on the fact that discrete space BNs are less prone to under-training occurrences compared to the continuous space ones [16].
