If you live in the DMV and want a simple to make and fairly cheap costume that will be recognizable to everyone, then you are in luck my friend. I have the costume for you. If you don't, but like to confound people and/or spend a lot of time explaining your costumes, then you, too, are in luck! Although, I'm pretty sure these basic steps can be adapted to fit to many different cities' transport systems.

Here's what you need for a basic metro station costume:

brown sweatshirt or long-sleeved shirt
brown pants
brown shoes
white iron-on letters (preferably Helvetica) OR white fabric paint OR white marker OR white-out
an iron OR a paintbrush for lettering (depending on above choice)
felt OR construction paper (two shades of grey plus orange, red, blue, green, yellow [or silver if you are forward thinking] depending on the station you want to be)
brown matte board OR any cardboard plus brown construction paper
plain white paper
glue gun or tape
craft glue
needle/thread (optional)
cotton batting (optional)

Step 1: Metro Station Shirt

Those of you who have traveled in DC's distinctive transport system will recognize the rounded, repetitive concrete pattern that's inside every underground station. We're going to replicate a suggestion of that architectural detail as shoulder pads.

Cut out 2 large rectangles, about 4x7 or so, out out of the darker grey felt. Out of the lighter grey felt, cut out three smaller rectangles (rounded at the corners) and place and regular intervals on top of the large rectangle (see illustration.) Sew or glue into place.
Sew or glue the large rectangles onto the shoulders of the sweatshirt. I sewed all my pieces, and I added some cotton batting underneath to add a little dimensionality.
Cut a circle of felt (according to your station color) and sew or glue it just below the grey shoulder pads on each side.
Use your iron-on letters to spell out the station name below the circle on the sleeve on each side. Alternatively, paint on the letters with white fabric paint.

Is certainly possible that you could go EVEN MORE super cheap, quick, and easy with all these steps using construction paper, tape and white-out.

Step 2: Metro Station Hat

Cut five squares of brown matte board. You could also cut them of any other kind of cardboard and paint brown or cover with brown construction paper. You should calculate the length of the sides by determining the diameter of your head at the widest point. Do this by measuring a form fitting hat you own, or measure the circumference of your head, calculate diameter and add an inch or so.
Use a glue gun or duct tape (or whatever) to attach the five squares together on the inside into an open-ended cube.
Cut 4 Helvetica capital Ms and 4 stripes out of white paper and glue to the four side squares as indicated.

Step 3: Build Your Rail System

You can do this costume as a single station, or add as many stations - and train cars - as you like! For our costume, we were the opposite ends of the Orange line, with a train car to run between us. Here are some other things you can add to your costume.

Additional people...more stations! Five different color lines! See WMATA for more info.
Add trains! If you want your dog to dress as a train car, here's how to make costume for your dog. (If you want a human to be a train car you'll have to figure that one out on your own.)
Audio! Someone carries a push button recording playback of various Metro soundbytes, such as "PLEASE STAND CLEAR OF THE DOORS!" or "HEY...IS THAT YOUR BAG? IF YOU SEE IT, SAY IT!"
Props! Carry around a suspicious package.

Those are just a few of the ideas we used. Experiment and enjoy!