Here is a nice tunnel for your layout. It is 17" long, 12" wide at the base and over 9" tall. The opening for the train is about 8" tall 6.5" wide. Lionel made it for Standard Gauge, but advertised it for standard or O.
I always liked the old Lionel logo at the keystone just over the top middle of the entrance each side.
CONDITION: C6+. Paint chipping on the raised peak in the center. Two story house on one side is missing the roof. Other paint rubs and nicks here and there--all pictured. Otherwise, looks good and ready to use.