Train's last Sensation.
--The Boston Commercial Bulletin contains a number of letters addressed by Geo. F. Train to the London American, and giving a detailed account of his experiences in an English ‘"sponging house."’ These letters from Mr. Train interfere considerably with the supposition which prevailed in this country, when the news of his arrest first arrived, that it had been caused by the British Government in consequence of a seditious speech. It turns out that ‘"Young America has been imprisoned for debt."’