There is an Amtrak station nearby to Baltimore-Washington International Airport (both are code BWI in respective travel systems). There is a 2.1 mile, 41 minute estimated walk duration open-to-weather footpath between these transit nodes; they are not exactly co-located. The BWI webpage only states the periodicity of a shuttle (1 per 10 minutes in the midday) not the duration of the shuttle leg.
BWI Marshall Airport offers free, frequent, and convenient shuttle service between the MARC/Amtrak station and the BWI Marshall Airport terminal. Shuttle Service operates 24 hours a day, 365 days a year. Shuttles run approximately every 10 minutes, except between the hours of 1:00 a.m. and 5:00 a.m. when service operates every 25 minutes.