Steam outline and center-cab electric are the two basic shapes for classic tinplate locomotives. The third option is a boxcab electric, but these are not very common and mostly quite small, like the Lionel 8E. McCoy used to make some nice larger boxcabs, but we are talking about 20-30 year old locomotives here that are available only on the secondary market.
The center-cab electrics are divided into two styles - New York Central, like your 408E and the 318E, and the Milwaukee Road Bi-Polar style, exemplified by the Lionel 381E and the much smaller 10E. If you want an electric that doesn't look like the 408, either of those might be a good choice.
My personal preference for running freight trains is a 392E, a 4-4-2 Lionel steamer. It's the most realistic and, in my opinion, the best looking of the Lionel steamers. Mine is an older MTH repro in gloss black with nickel trim. It has also been made in black crackle finish, which is very cool looking. Another good thing about a 392E is that it's a good size to go with either 500 series (smaller) or 200 series (larger) freight cars.
Another great choice for a steam engine is the American Flyer "Brass Piper." MTH and "LCT" (actually also MTH) have reproduced this one. It's also a 4-4-2 and is a gorgeous piece with its brass piping on the side.
Sorry, I don't have pictures of most of these locomotives. Here's one shot showing the front portion of a 392E.
, orange
