。
She trained to become a doctor.
Literal
She [topic-は] doctor [to-に] become training [object-を] received.
訓練を受ける ('to receive training, undergo training') is the standard collocation — note the verb is 受ける ('receive'), reflecting a Japanese tendency to frame the trainee as the recipient of training delivered by an institution rather than as an active 'studier.' 医者になる訓練 uses 医者になる ('become a doctor') as a noun-modifying clause attached directly to 訓練 — the whole unit reads 'training to become a doctor.' Common pattern: dictionary-form clause + noun, no relative pronoun needed.