Adriatic,
The Polar Express set has always had the same "Berkshire Jr." that came out around the same time as the "Hudson Jr.". The Berk continues in production. The Hudson Jr. hasn't been released in years.
The Berkshire Jr. has come with two motor variations (large & small) and two body configs (with and without handrails).
As far as I know, the chassis and wheels have been the same o all of them. They appear to be a little "chubby" for their size, which when combined with the longer wheel base may be causing the issue here.
Jon