Train

They are both natural, but the first required a teacher and the second one did not.

So the intransitive 'train' have a strong sense of self-taught, compared to the transitive 'train'?

Yes. Of course, naming the sport/skill would probably influence the sensation.

Good.

Then, do you think it's safe to say that the intransitive 'train' is almost, if not exactly, the same as 'train oneself'?

That seems reasonable, I suppose.

They mean basically the same thing, and might be used (crudely) interchangeably, but there is the subtlest shade in nuance and they would typically be used in different contexts:

"He was trained as a singer, but decided to become a pianist." You could also say, grammatically (but crudely): "He trained as a singer but decided to become a pianist.", but this particular pattern favors "was

Mister MicawberThat seems reasonable, I suppose.

Good. Thank you, MM.

MM, anon below you says:

Anonymous The "under ___" pattern favors "trained as a singer."

Would you disagree?

Yes, of course. I usually disagree with him. He is usually frighteningly prescriptive.

Oh, is the anon the same person you've seen somewhere else before?

Yes. As a moderator, I can read his IP address.