Critical Cars, Traffic Research

Critical Cars: AC&F “Flexi-Flo” PD 3500

The American Car & Foundry’s 3500 cubic foot, pressure differential covered hopper (PD3500) built for the New York Central in the mid-1960s was one of the most distinctive covered hopper designs of all time. Only 220 were built, but their long service life, distinctive graphics and concentration in the populous Northeast have made them a must-have model for many. Those of us who need these cars are fortunate that Rapido has recently released versions of all three lots in HO scale.

The models are excellent, a quick tune up. For me modeling them only a few months old, a shot of Dullcote and some light weathering and they are ready for the road. While there isn’t much more to say about the models, how these cars came into existence and the role they played in helping railroads adapt to changing times is an interesting story.

Rapido’s models are a spot-on match. They just need a good weathering job to get on the layout.


In the 1960s, railroads were seeing their traffic erode on all fronts due to increased competition from alternate modes of transportation. This trend even began to attack bulk commodities that were once considered “safe”. One of these commodities was cement. In the Hudson Valley around Albany, there were nine cement mills that supplied much of New York and New England’s cement. New York Central served seven of these directly. After the opening of superhighways across this territory and the development of the pneumatic dry bulk trailer, these plants diverted significant tonnage to truck. The NYC was looking for a way to get this traffic back and the Flexi-Flo concept was the result. 

Central’s president, Al Perlman, summarized the Flexi-Flo development at an ICC hearing in October of 1965,

…a few years ago [cement] moved almost exclusively by rail in covered hoppers. The great convenience of jobsite delivery by truck, and the difficulty and expense of transfer from rail hopper to truck storage, thence to truck for jobsite delivery, led to widespread decentralization of the cement industry, despite the fact that centralized cement production offered great production cost advantages. If these production cost savings were to be passed along to the consumer, a new transportation tool had to be found.

The Central met that challenge by the development of the Flexi-Flo car, a covered hopper of 125-ton capacity with a pressure differential (PD) device. The car moves from production point to terminal where it is unloaded, as the cement is needed, into pressure differential-equipped trucks. Differences in pressure within the truck’s hopper and the rail car hopper permit loading the truck in a matter of minutes. Flexi-Flo reduces the cost of basic transportation, eliminates the need for storage in transit, and permits the ultimate consumer to share in the benefits of all these cost reductions, including that of centralized production of cement.

Excerpt from January 1966 Central Headlight. NYCSHS collection, used with permission.

While the concept of team tracks is as old as railroading itself, the NYC innovated with Flexi-Flo by creating a full service business model tailored to the specialized needs of dry and liquid bulk shippers, combining new concepts in cars, terminals and operations making it much more user-friendly and economical at the same time.

The Cars

New York Central specified a car with a 315,000 pound gross weight (315K) to take full advantage of the heavy-duty capability of its core mainlines. The cars had a load limit of 245,200 lbs., allowing them to hold six truckloads of product each – a critical factor when trying to win business back from the highway. The specific gravity of powdered cement dictated a 3500 cubic foot capacity. To emphasize how far ahead of the times NYC was consider that the standard railcar gross weight was 220K, there still isn’t a national 315K network in 2020 and some short lines and regional railroads are still struggling to get their bridges upgraded to 286K.

The cars were delivered to NYC in four batches as outlined in the roster below. They each differed slightly from each other. The most obvious spotting characteristic were the vertical side ribs on the first batch of 25 cars. One has to look closely to spot the differences between the next two lots. The final NYC group was made up of five 3600 cubic foot tank type covered hoppers built for BF Goodrich. They are included for reference but are a totally different car design.

NumbersQty.LotYearSide ReinforcementsHatch Dia.Wheel Dia.

With the exception of 7 cars built for AC&F’s Shippers Car Line lease fleet, these wound up being unique cars to the NYC. The 3500 cubic foot design had great economics, but the limited number of routes that could handle 315K cars probably led to Penn Central’s decision to specify a 3000 cubic foot, 263K limit design for the next batch of PD cars in 1974.

From left to right, cars from the first, third and fifth orders of Flexi-Flo covered hoppers at Bridge One in Cleveland, Ohio in the late 1970s. Note the vertical stiffeners on the first car. The fifth batch was PC’s 1974 order of North American Car PD3000s. PC would also add 2600 cubic foot Airslide covered hoppers to the Flexi-Flo fleet as well. George Elwood photo, used with permission


Flexi-Flo cars at Framingham, Mass. circa 1965. Note the cement streaks starting to show up, new cement cars get dirty fast. This was the photo I used as a reference for weathering my cars. Tad Arnold photo, Bob Arnold collection (used with permission)

Flexi-Flo terminals consisted of one or more tracks to hold cars while they were unloaded, a driveway parallel to the track wide enough for trucks to turn around, pneumatic equipment to pump the product, a truck scale and an office. The first Flexi-Flo terminal was opened at Big Four Yard in Avon, Ind. near Indianapolis in May of 1964.

Ceremonial opening of the Syracuse Flexi-Flo terminal. Central Headlight – courtesy of the New York Central Historical Society.

Through the Penn Central merger, NYC opened 10 additional terminals, in the following order (dates are approximate):

  • Rochester, N.Y, – 9/64
  • Buffalo, N.Y. – 1/65
  • Syracuse, N.Y. – 3/65
  • Framingham, Mass. – 9/65
  • Louisville, Ky.
  • North Bergen, N.J. – 4/66
  • Charleston, W.Va.
  • Boston, Mass.
  • Hammond, Ind. – 7/67
  • Cleveland/Collinwood, Ohio – 11/67

Generally speaking, the railroad opened the first terminals to create a distribution network for Hudson Valley cement. They then pivoted to handle general liquid and bulk commodities, starting with the Louisville terminal.

A Framingham switcher pulls empty autoracks from the auto distribution terminal back to the yard, June 1966. In the center of the photo is the Flexi-Flo cement terminal. Leroy Dozier photo, George Elwood collection, used with permission.

The Framingham terminal was located around the wye with the Milford Branch and on the site of the freight house, which was demolished to make way for this project. A 1966 USGS aerial photo shows 20 cars spotted there. If those are all Flexi-Flo cars, and based on period photos I believe they are, that is 1/5 of the fleet at that time. Obviously Framingham was one of the busiest terminals in the network and probably the top destination of the PD3500 fleet. The terminal continued to serve cement customers into the middle of the 1980s when bankruptcies and mergers in the cement industry led to major changes in the distribution network. Any remaining activity was consolidated with the Boston Flexi-Flo operation at that time.

Framingham Flexi-Flo terminal, March 9, 1966. USGS aerial photo.

The Boston terminal was located off the Sears Lead on the north side of the MassPike and was built in concert with the total reconstruction of Beacon Park Yard brought on by the Turnpike’s construction. In later years, as carload freight in and out of Boston declined, the terminal was moved into the middle of Beacon Park Yard. With the closure of Beacon Park, CSX moved it again to Westborough, under their Transflo brand.

Beacon Park freight terminal April 9, 1969. The Flexi-Flo yard is filled with a mix of covered hoppers and tank cars. It looks so full that the circulatory truck access has been blocked. Notice that there are 49 boxcars spotted at the freight houses, an indication that the freight forwarder business was still brisk. USGS aerial photo.


The NYC needed the Flexi-Flo cement service to succeed so it could serve as a example of what the new concept could achieve. Given what was at stake, these cars received priority handing, both loaded and empty to ensure fast car cycles. Loads moved east on BA-6 from Selkirk, N.Y. to Framingham, arriving late afternoon/early evening. Most likely, cars were spotted overnight to not disrupt the unloading process during the day. Empties would move out the following morning on a westbound extra to Selkirk. Feeders from Selkirk to Kingston and Hudson connected the cement mills to Selkirk Yard.

NYC train BA-6 from Selkirk to Boston at Ashland, Mass. June, 1966 carrying a block of Flexi-Flo cement loads three cars from the headpin. All the cars visible here are to be set off at Framingham. Donald Haskel photo, used with permission.


The Flexi-Flo cars served a long life with PC, CR. Conrail transferred some cars to Merchants Despatch before selling off the fleet. Some users in later years included Pfizer/Specialty Minerals for limestone out of both Canaan, Conn. and Adams, Mass. as well as Dragon Products at Thomaston, Maine for cement. The last cars aged out of interchange service in 2016. Recently Conrail 80019 (built as NYC 885739) was donated by CSX to Conrail Historical Society for preservation at the Danbury Railroad Museum in Connecticut. This is thought to be the only surviving Flexi-Flo hopper – lasting because it was converted to a scale test car.

The Flexi-Flo brand was continued by Penn Central and Conrail and expanded to other locations in later years. Other railroads followed suit with branded, full-service bulk team track operations to the point where a network of such terminals is considered a necessary service for all the Class I railroads – underscored by CSX’s purchase of liquid bulk trucker Quality Carriers this week. This is the true legacy of Flexi-Flo and a lasting testament to the creativity of NYC under the Perlman administration.


Savage Services bulk rail transload, Hammond, Ind. – ex-NYC Flexi-Flo terminal present day. Google Earth image


The Story of Flexi-Flo, Central Headlight, January 1966

Flexi-Flo Sales Brochure (from mid-1966)

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