Understanding the back of your postcards
On postally used postcards, the cancellation may have value to stamp collectors (philatelists), and can make the cards more valuable and interesting to postcard collectors. It's always fun to know more about the history of your postcards and where they have traveled. Information is provided below about some specific types of cancels to look for.
Discontinued Post Office Cancels: A discontinued post office is one that has closed down for some reason. The town served by the post office may have become too small, or the post office may have been combined with another to say money. Generally, the shorter the life span of the post office and the smaller the population, the greater the value of the discontinued post office (DPO) cancel. To check for a DPO cancel, look up the post office in a zip code directory. If it is not there, you have a DPO cancel.
Rail Post Office Cancels: At one time a post office car would be hooked up to a train and the clerks would sort mail as the train carried them between stations. The mail from these cars usually carries the names of the stations at each end of the line, plus the letters R.P.O. Cancels usually contain RPO in the town mark and RMS in the killer (the part of the cancel that covers the stamp). Visit the National Postal Museum's Mail by Rail page for more information about rail post offices. Also see the Mobile Post Office Society's RPO Postmark page.
Trolley Post Office Cancels: A number of cities, including Baltimore, Brooklyn, Boston, Chicago, Rochester, and St. Louis, used trolley cars to pick up mail, which was sorted and cancelled on the car. There were a number of car lines in each city, with different cancels, so of which are rare and valuable. Trolley Post Office (TPO) cancels will often have the names of the streets serviced or the communities within the town that are being served. The cars serviced the letter boxes on their route and had the mail sorted by the time they reached the main post office.
RFD Cancels: These are cancels for mail that the RFD delivery man picked up from one postal customer and delivered to another customer on the same route. He cancelled the stamp with an indelible pencil (purple), sometimes putting his initials and the date. On busy routes, the postman would invest in a rubber stamp canceling device with the letters RFD and a town name and a date, or the words Rural Route.