MAR Online Store › Museum of the American Railroad Big Boy 4018 Cap - Adult Size Image 1 of 1 Museum of the American Railroad Big Boy 4018 Cap - Adult Size $19.99 Wear this cap and show off UP Big Boy 4018! Adult size cap with adjustable Velcro tabs in back. Add To Cart Added! Wear this cap and show off UP Big Boy 4018! Adult size cap with adjustable Velcro tabs in back.