I highly recommend doing this mission whenthe doctor at the memory loungetells you to do it. If you are past that, here we go with the freedom trail.

At your quest marker you will see a red path thats leads to the railroads destination, along the way eight codes can be found, I have marked them all here. on the way there is bandits and super mutants, be prepared.

The first combination is; 7 - A

The second combination is; 4 - L

The third combination is; 2 - A

The forth combination is; 6 - O (the letter O not the number 0)

The fith combination is; 3 - I

The sixth combination is; 5 - R

The seventh combination is; 8 - D

The eighth combination is; 1 - R

Once you have reached the end of the path you should be at the Old Church, go on in and enter it.

Once you enter the church you'll need to look for the railroad directly, around the church is lots of feral gouls so be prepared.

Once you are in and have killed the first ammount of gouls, go to the balcony for some Nuka-Cola and other foods inside of a cooler, once you have done this proceed to the basement.

After killing multiple ghouls you'll encounter the code puzzle.

Once you've gotten to the puzzle you'll need to enter the code, the code decripted is;

RAILROAD, enter it and you will be in, congratulations.

I hope this guide helped, ( ^ 3 ^)