Baltimore & Ohio: Maine Lumber Expansion is the smallest expansion Winsome has ever produced, but it has a big effect on the base game.
This is part of Winsome's Essen 2010 set.
Winsome Games' 2010 Essen Set

This expansion is reported to open up the northeastern portion of the map - a portion that is often neglected in the base game.

This game adds one lumber counter to the game. This lumber counter is placed in Augusta. This counter is taken as a railroad company's entire build action for that round, it is however, free of other costs. The counter adds $10 times the current tech level to the income of the railroad company (provided the company has any Capital Equipment) and does not count against the Service Capacity limit. This counter acts as promised and does open up the Northeastern portion of the map to more gameplay. (Taken from Gameplay)