Here's a link to the street view of that address c2010, now the Merdien [Meridien] Hotel, only a few blocks from the Train Station [Hauptbahnhof]. There are similarities between the building then and now, but the differences (no windows shown on the right side of the post card, now balconies at every window, arches over the windows, etc.) would imply they have done a lot of renovating, or I'm looking at the wrong address.
[Note the marking around the window of the room where they stayed.]
Had a very nice room here last night – not too far from station on more of a residential street, and tourist hotels are high. Ours is more for Germans but proprietor spoke very good British English.
Received letter from Elizabeth (mailed April 4, received April 9) – only one received here. Said she'd heard from you and received two beautiful dresses for Catherine & Roberta. Will send letter on to you.
On way to St. Goar. Tomorrow we board the steamer down the Rhine to Cologne. All our very best love, Harold & Jean – Many American soldiers here. Generous servings of food. Frankfurt was bombed. [Last three sentences squeezed in at side and top.]