I like the Lionel Amtrak cab car and have the original. The new ones seem like they will be upgraded. However, there's a caveat- they don't look correct with any other passenger cars I own besides the Lionel Amtrak 18" Amfleets(I have 5 of these- 3 from the HHP-8 set and the 2-pack add-on). By extension, the 18" Lionel Amfleets don't look right with any other of my passenger cars besides themselves. Even if you aren't a stickler for exact scale proportions and run different length cars, it's like running scale a scale 3 bay covered hopper/50' boxcar/etc... with a semi-scale counterpart. Thus, I'd advise buying this car as long as you get some of the other Lionel 18" Amfleets to go with it. On the plus side, a consist of the Lionel Amfleets still looks decent behind any Amtrak loco- be it MTH, Lionel, Atlas, etc...IMO
Due to the unique size of the Lionel 18" Amfleets, I have tried pairing them with a multitude of different passenger cars to try and find a decent size match- even if it hasn't necessarily been another Amtrak car. Below is a list of 18" to 21" passenger cars with which I have paired the Lionel Amtrak Cab car. The closest matches are in bold. If you want pictures I'll provide them:
MTH:
19" Amfleets ex. Product # 20-64015 (now Atlas O Premier Amfleets)
70' Scale Aluminum Passenger Cars ex. Product # MT-6508
Golden Gate Depot:
21" Amfleet
K-line:
18" Amtrak Superliner II transition car
21" Amtrak Superliner II Coach and Sleeper
18" Amtrak Heritage Baggage
21" Amtrak Heritage Observation
Weaver:
80' (20") New Haven McGinnis Scheme Pullman Bradley
Atlas O:
21" Amtrak Horizons(Phase III- coaches and diner)
80' (20") Kansas City Southern Pullman Bradley
I think the reason the GGD is the the closest is because it is the most accurate/only scale representation of an Amfleet car on the Market, it's width and height are bigger, but closer, to the Lionel version. The other scale Amtrak cars, such as 21" heritage or Horizons, are bigger in general. While you'd think The MTH 19" Amfleets would be closest, their dimensions vs the Lionel 18" are so different that the MTH appear shorter and the Lionel appear longer.
Hope that helps