The Fissure Detector depends for its operation upon the fact that electric current flowing through a rail is compelled to pass around any fracture, inclusion, or separation in the metal. In the detector car, the power plant for the detector equipment consists of a specially designed double commutator 6,000-ampere, 2-volt DC generator directly coupled to a 50 hp engine. Current from the generator is carried by heavy busses to brush units located on either side of the car. In operation the car passes along the track at about 6 mph, with 2,000 to 3,000 amperes flowing through each rail. The current is sensed by the pickup unit, amplified and sent to a recording table, which consists of a moving roll of paper and nine pens (like a polygraph or lie detector). The top pen (landmark pen) is marked by the car's operator to indicate mileposts and other landmarks, the middle two pens record angle bars (rail joints). When the car passes over a fissure, and indication appears on the record. Three recorder pen relays are connected to the output of the amplifier and adjusted to different values of plate current, thus giving an indication of the size of the defect. Other relays are provided to operate paint guns and the defect area is automatically marked with a spot painted on the rail. The operator notes the record, sees the spot painted on the rail, and stopping the car, backs up for a hand test, applying 1,500 amperes to the suspected spot. Examination of the spot with a galvanometer can determine the size of the fissure within a few percent. The car can also find split heads, horizontal fissures, compound fissures, pipes, cracked webs, broken bases and other defects.

The car in front of it has C&NW reporting marks, but also has Genesee & Wyoming painted on the letterboard. It appears that it was numbered C&NW 404, later C&NW 440 and named "Philip R. Hastings".

As built, the RDC3 consisted of a 17 foot Railway Post Office, a 17 foot express room, and a 40 foot, 49-seat passenger section. Some RDC3's were built without RPO apartments, but used an identical carbody to those so equipped. Of the five RDC3's originally owned by the New Haven, #'s 126 and 129 migrated to Pennsylvania DOT ownership, where they were converted to all-coach layout. At first they were assigned to Pittsburgh PAT service, later they sent to SEPTA service in Philadelphia. Still later they were sent to Morrison-Knudsen, converted almost back to the original configuration, but without the RPO apartment, and sent to Alaska.

Another New Haven RDC3, the 130, having been converted to a self-propelled track inspection car, found itself in the employ of the C&NW. The carbody modifications were numerous. On the right side, the RPO door and window were retained, but the baggage door was filled in using matching flutes below the belt rail, replacing the opposite side's RPO window with stainless sheet above the belt. The second and third passenger windows were removed and and relocated to the left side in the former location of the baggage door. That displaced door frame was shifted forward to the position of the left side RPO door, which was discarded. On the passenger end, all cab windows, both end and side, were deepened to within a foot of the floor to provide the observation room. Although the letterboard duplicated the graphics of the original 1950 RDCs, the sides of the ex-NH 130 featured two sizes of C&NW logos, and the car's front end was painted with green and yellow diagonal stripes.

As built, the RDC3 consisted of a 17 foot Railway Post Office, a 17 foot express room, and a 40 foot, 49-seat passenger section. Some RDC3's were built without RPO apartments, but used an identical carbody to those so equipped. Of the five RDC3's originally owned by the New Haven, #'s 126 and 129 migrated to Pennsylvania DOT ownership, where they were converted to all-coach layout. At first they were assigned to Pittsburgh PAT service, later they sent to SEPTA service in Philadelphia. Still later they were sent to Morrison-Knudsen, converted almost back to the original configuration, but without the RPO apartment, and sent to Alaska. Another New Haven RDC3, the #130, having been converted to a self-propelled track inspection car, found itself in the employ of the C&NW. The carbody modifications were numerous. On the right side, the RPO door and window were retained, but the baggage door was filled in using matching flutes below the belt rail, replacing the opposite side's RPO window with stainless sheet above the belt. The second and third passenger windows were removed and and relocated to the left side in the former location of the baggage door. That displaced door frame was shifted forward to the position of the left side RPO door, which was discarded. On the passenger end, all cab windows, both end and side, were deepened to within a foot of the floor to provide the observation room. Although the letterboard duplicated the graphics of the original 1950 RDCs, the sides of the ex-NH #130 featured two sizes of C&NW logos, and the car's front end was painted with green and yellow diagonal stripes.