Baltimore and Ohio 4500 is a 2-8-2 "USRA Light Mikado" steam locomotive built by the Baldwin Locomotive Works in Philadelphia, Pennsylvania in July 1918 for the Baltimore and Ohio Railroad (B&O) as a member of the Q-3 class.

The locomotive hauled freight for the B&O until retirement in August 1957 and was donated for display at the B&O Railroad Museum in Baltimore, Maryland.

It is the sole surviving Baltimore and Ohio Mikado type steam locomotive.

While it has mostly remained as built mechanically, it received some of B&O's distinctive cosmetic changes throughout its service life, but the locomotive retains the original tender and trailing truck.

Following America's entry into WWI, the USRA nationalised the nation's railroad system in the interest of ensuring the most efficient operations possible.

Like the other USRA designs, #4500 was sturdy, functional and popular with maintenance and locomotive crews.

4500 operated on the B&O's Ohio Division mainly hauling freight until it was retired from service in 1957, and it was donated to the B&O Railroad Museum in 1960 where it is placed on static display today.