On January 1, 1926, when Paul Kiefer took over as Chief Mechanical Engineer of Motive Power and Rolling Stock of the New York Central System, the NYC was the nations largest railroad. At that time, its passenger business had grown to a point that many of its main line trains had to be operated in sections because the Class K-5 Pacifics assigned to passenger service could only haul a maximum of 12 cars. It was apparent that a new passenger locomotive was needed, yet no decision had been reached to solve the passenger power crisis.

Kiefer quickly made a decision to proceed with an experimental 4-6-4 locomotive and selected the American Locomotive Company to build it. He followed the example of Lima's William E. Woodard and designed a locomotive with a large grate area and a four wheel trailing truck to support it.

Kiefer's design looked much like a Class K-5, Pacific. It was only 5 inches longer and had the same cylinders (25 X 28) and drivers (79") as the 4-6-2's, but the boiler pressure was increased to 225 psi. The new 4-6-4 locomotive had a slightly larger boiler evaporative heating surface, 24 sq. ft. more of firebox and 14 sq. ft. more of grate area. This new design could produce much more steam per square foot of heating surface and needed a much larger superheater to maintain the same steam temperature as the K-5's. The total weight was about 41,000 lbs more, and with no increase of weight over the drivers. An increase of 3820 lbs in Tractive Effort was obtained with the increase in boiler pressure.

The American Locomotive Company delivered the first Class J-1a (road number 5200) on February 14, 1927 and it was thoroughly tested on the main line between Albany, NY and Syracuse, NY.

After the testing, Paul Kiefer went to see Pat Crowley who was the president of the entire NYC system. He asked Crowley what name the new locomotive should have, Crowley thought for a few minutes and then said "let's call her the Hudson, after the Hudson River" and the name stuck.

In 1914, the Montreal Locomotive Works built six Class X-10a, 4-6-4T locomotives for the Grand Trunk Railroad, which later became part of the Canadian National Railway. The "T" stands for "tank" which means that these locomotives have a water tank and coal bin integral with their main frames rather than separate tenders. They were given the name "Baltic Tank" and were used for commuter service in the city of Montreal. Originally, these "Tanks" were given GTR road numbers 1540 through 1545 and later were renumbered with CNR numbers 45 through 50.

Class X-10a Specifications

Wheel Arrangement:	4-6-4
Length:	50' - 2"
Drivers:	63" dia.
Weight on Drivers:	146,000 lbs
Total Weight:	275,000 lbs
Grate Area:	47 sq ft
Cylinders:	(2) 21" dia. x 26" stroke
Cylinder HP:	1588 HP
Boiler Pressure:	210 psi
Tractive Effort:	32,487 lbs
Water Capacity:	2,900 gals
Coal Capacity:	5 tons

The American Locomotive Company built a single "Baltic Tank" locomotive, in 1922, for the Tennessee Coal, Iron & Railroad Company. This company used it for a company commuter train which provided transportation for its workers from the city to the mines.

The Central Railroad of New Jersey had the Baldwin Locomotive Works design a 4-6-4T for it and in 1923 took delivery of six of the "Baltic Tanks" for use in its commuter service.

The Canadian Pacific Railway built three 4-6-4 tank locomotives in its own shops in 1910/1912. These "Tanks" were designated Class T2a and assigned road numbers 5991 through 5993.

There are three surviving 4-6-4T locomotives, all from the CNR, number 46 at the Centre d'Interpretation Ferroviaire de Vallee-Jonction, QC; number 47 at Steamtown NHS, Scranton, PA and number 49 at the Canadian Railway Museum, Delson, QC.