Welcome to the forum!

With that large a layout, multiple power connections to the track are a must, one every 3-4 feet or so. As has been stated, clean track, wheels, and roller pickups are a necessity for proper operation.

As for power, if you are powering accessories from the same transformer, you probably have it loaded to the limit. The locomotive may also be at its' limit for such a long train also. An upgrade to a larger transformer is definitely needed, as well a larger locomotive. The older postwar KW and ZW transformers will serve you well, and you can use them to power the layout into the future, even if you upgrade to a command layout.

Larry