Redwood beams from Los Angeles' Old River Station--our pre-1939 train station--have been unearthed, per this Daily Breeze article. The location is Spring Street, near Dodger Stadium. River Station was built in 1875 and the first steam engines to use the station arrived in Los Angeles via ship. (Phineas Banning had already built a short rail line from the harbor to downtown, about 22 miles long, so there were tracks for the new engines to ride)
Archaeologists have stumbled upon the remains of a roundhouse. For those non-parents who don't know their Thomas the Train icons as well as they should, a roundhouse is a building that faces a giant turntable. The roundhouse has multiple ports, or service bays, for train engines, and the turntable points those engines to the right service bay. This wonderful picture from Shorpy is of a roundhouse in Chicago in 1942. The service bays are filled with engines delivered by the turntable.
The redwood beams found in Los Angeles were actually part of the massive turntable. The Times has more information; they say that sonar imaging led archaeologists to the finds, which include artifacts from the roundhouse itself, and that finds were made in three locations.
The Times also points out that tracks existed at this spot as recently as the 1990s. The site used to be called the Cornfield but now known as Los Angeles State Historic Park, thanks to a 2001 purchase of the property by the state.
Incidentally, ERHA--The Electric Railway Historic Association of Southern California--hosts a fabu site of Los Angeles' 19th century railroad history. Anything you want to know--more than you want to know--they've got it covered. ERHA even sells coffee cups, mouse pads, bags, etc. with LARy (Los Angeles Railway) logos, here.