@Jason Zhang welcome to BP. As others have stated, you won't get any DC commuters renting a multi-family in Hagerstown. There are few things to consider when looking in Hagerstown
1. Stay away from anything that's not in the North End. You'll be hard pressed to find a quality place and quality tenants in any other parts. You'll get C class units and C-D class tenants in any other area.
2. The City is against you. The city has been trying to crack down on slumlords in the past few years so they're really strict. You'll have to pay a yearly $75 license fee per unit. If the unit is built before 1979 you'll need to pay for lead paint certificates and inspections. The city will inspect your unit every 3 years or at every tenant turnover after that and they ALWAYS find something wrong that you'll need to fix. If the police are called to your units, the city will fine YOU. If trash is on the sidewalk, the city will fine YOU. You really need to screen your tenants well and make sure to have a good management company
That being said, you can find some good deals in the north end if you know what you're looking for. Hope this helps