Error: System.NullReferenceException: Object reference not set to an instance of an object.
Basicly means theres a an object mentioned in the the world file that does not exist in another file.
The version of OR you are using 1648 is now quite old versions around this time did have a problem with the start signal (near the signal box from memory) at Seymour. The OR devs did explain to me what the error was but I cannot rememeber it now.
Try the latest version found at http://www.openrails.org/experimental.html
There are a number of options now OR has that makes the Seymour route, one allows all objects on the tiles viewd to be loaded regardless of LOD value, the stops objects from appearing right in front of you. The second one can now increase the viewing distance, this one can be tuned to the frame rate.
This latter item absolutely makes Russels Seymour route as one can then see the hills approaching and on either side of the route.
When you come over the hills around Donnybrook one can see the range north of Wallan.