When Conrail took over operations in 1976, they retired many of the first generation diesels that had been in hump yard duty, leading to a major shortage of those engines. In a Railroad Model Craftsman article, Ken Goslett mentions how the hump yard at Allentown, PA, went from using Central Railroad of New Jersey RSD-4's to leased Bangor & Aroostook Geeps and Pittsburg and Shawmut Railroad switchers. By 1978, however, Conrail's "real" hump engines had become their former Penn Central SD38's. Still, a pair of painted-out Penn Central SD9's worked the Allentown hump for years after the leased power left. Over at the former Reading Company hump in Rutherford, PA, a pair of GP30's did duty on one end until the hump closed.

In 1978 and 1979, Conrail decided to construct a fleet of slugs using worn-out Alco RS-11, RSD-12, and RSD-15 units. Conrail removed the diesel engine, generator, and cab from the original units. The underframe, trucks, traction motors, and blowers remained. The hoods were chopped down and patched up. Details of individual slugs varied depending on the original unit and what the shops had on hand. Those slugs that came from four-axle ALCo units were classed as MT-4's (Motor Trailer 4-axle).

By May 1999, only 4 MT-4's remained on roster. These units were transferred to CSX and Norfolk Southern on June 1, 1999.

Comprehensive MT-4 Roster

The roster below covers those units operated by Conrail from the creation of the first unit in 1978 to their retirement from Conrail, along with first disposition. The roster includes each unit's first disposition when known. A special thanks to the Diesel Shop, X2200 South, the Conrail Historical Society, and the many Conrail enthusiasts who helped put this roster together.

CR #   Reblt   Former    Prior     Previous   Previous   Removed   Disposition
==============================================================================
1000   01/79   CR 7634   PC 7634   PRR 7634   PRR 8634   10/03/95  OHCR 1000
1001   02/79   CR 7625   PC 7625   PRR 7625   PRR 8625   10/03/95  OHCR 1001
1002   02/79   CR 7636   PC 7638   PRR 7638   PRR 8638   10/03/95  OHCR 1002 
1003   03/79   CR 7638   PC 7636   PRR 7636   PRR 8636   05/31/99  CSX 1003
1004   03/79   CR 7646   PC 7646   PRR 7646   PRR 8646   05/31/99  NS 1003
1005   03/79   CR 7607   PC 7607   NYC 7607   NYC 8007   10/03/95  OHCR 1005
1006   04/79   CR 7608   PC 7608   NYC 7608   NYC 8008   05/31/99  NS 1001
1007   04/79   CR 7654   PC 7654   PRR 7654   PRR 8654   10/03/95  WW 1007
1008   04/79   CR 7620   PC 7620   PRR 7620   PRR 8620   10/03/95  
1009   05/79   CR 7618   PC 7618   PRR 7618   PRR 8618   10/03/95  
1010   05/79   CR 7603   PC 7603   NYC 7603   NYC 8003   10/03/95  
1011   05/79   CR 7606   PC 7606   NYC 7606   NYC 8006   10/03/95  
1012   06/79   CR 7619   PC 7619   PRR 7619   PRR 8619   10/03/95  
1013   06/79   CR 7648   LV 7648    LV 8648   PRR 8648   10/03/95  
1014   06/79   CR 7653   PC 7653   PRR 7653   PRR 8653   10/03/95  
1015   07/79   LV 7642   LV 7642    LV 8642   PRR 8642   10/03/95  
1016   07/79   LV 7641   LV 7641    LV 8641   PRR 8641   10/03/95  
1017   07/79   LV 7643   LV 7643    LV 8643   PRR 8643   10/03/95  
1018   08/79   CR 7630   PC 7630   PRR 7630   PRR 8630   05/31/99  CSX 1018
1019   08/79   CR 7645   PC 7645   PRR 7645   PRR 8645   10/03/95  
1020   08/79   CR 7647   PC 7647   PRR 7647   PRR 8647   10/03/95  WW 120
1021   09/79   CR 7605   PC 7605   NYC 7605   NYC 8005   10/03/95  
1022   09/79   CR 7650   PC 7650   PRR 7650   PRR 8650   02/28/96  
1023   10/79   LV 7640   LV 7640    LV 8640   PRR 8640   10/03/95