Aside from the ten T26E1 pilot vehicles, a T26 with an electric transmission was also built, and shipped to Fort Knox for trials. As of April 10th, 1945, it travelled 1200 miles, with 224 hours and 38 minutes of operation time. The electric transmission is described to have "stood up very well", not requiring excessive maintenance.
In October of 1944, a wooden mock-up of the T26E1 with a 105 mm howitzer is completed. In January of 1945, the project is formally approved, at which point the vehicle is renamed T26E2.
A 90 mm gun with improved ballistic performance, designated T15, was ordered, with 100 units recommended for procurement on December 21st, 1944. The gun was originally designed for one-piece ammunition, but it was uncomfortably long. A variant of the gun with two-piece ammunition was also developed, designated T15E2. A pilot vehicle was sent to Aberdeen for a test firing, and shipped overseas afterwards. Tanks carrying the T15E2 were designated T26E4 on March 1st, 1945, and 1000 such vehicles were authorized in lieu of M26 tanks. Characteristics of the vehicle were expected to remain the same, except for an increased in mass of 3500 pounds. Drawings for the production model and a second pilot vehicle were nearly complete on April 10th, 1945, with the first such vehicle scheduled for completion on May 15th, 1945.
A replacement of the T15E2 90 mm gun was ordered by Major General G.M. Barnes to obtain a more desirable balance condition and use one-piece ammunition. The gun was mounted further forward in the turret to free up space in the fighting compartment, requiring the use of an equilibrator system to correct the 80,000 pound imbalance. Approximately 40 of the new rounds would fit into a T26E4 tank with a redesigned ammo rack. A pilot vehicle was scheduled for completion on October 1st, 1945.
The success of the M4A3E2 "Jumbo" Sherman led to research into an equivalent modification of the T26 tank. This modification had the effective armour of 8 inches in the front, as well as a new equilibrator, to offset the heavier gun mantlet. The vehicle's weight was estimated to be 97,000, but the tracks were widened with 5 inch wide connectors, making the vehicle's ground pressure 11.5 psi. The width of the tank with these tracks was 143 inches, 124 inches for railroad transport. The recommendation to perform this modification of the T26E3 and name it T26E5 was submitted on January 18th, 1945, and approved on February 8th. On March 29th, 1945, it was recommended that the thickness of the front armour be increased further, and 27 of these tanks be produced. The new T26E5 would have:
- A 6 inch thick UFP positioned at 46.5 degrees
- A 4 inch thick LFP positioned at 54 degrees
- An 11 inch thick unsloped gun mantlet
- Increased equilibrator capacity
- Increased thickness of the front turret ring splash guard
- Decreased width of the rear portion of the hull escape hatch doors to eliminate a turret weak spot
The weight of the vehicle increased to 107,000 pounds, and ground pressure grew to 12.1 psi. The vehicle was thought to be capable of speeds that could seriously damage the two front bogies on rough terrain, and modifications to the shock absorbers, torsion bars, and wheel bearings were designed, just in case. However, these changes did not make it to the production line, as it was decided to perform them on an individual basis.
These changes made the tank unwieldy. The only bridge capable of carrying it would be the M4 Floating Bridge, at a current speed of less than 5 miles per hour. The bridge, only 148 inches wide, could not safely accommodate a T26E5 when extended track connectors were uses. A T26E5 was built and delivered to the Aberdeen Proving Grounds in July of 1945.
In 1946, the design was scheduled to be upgraded to use a new 105 mm gun (not a howitzer), with a new transmission, a 750 hp Ford 12-cylinder engine, and a longer chassis. It is planned that the production of this new tank will be possible without significant changes to the T26 production process.
A 155 mm gun was also planned, capable of penetrating a 6 inch armour plate at 2000 yards, but also capable of firing a 95-lb HE shell, as well as a gun "of greater power and velocity than the 105-mm. gun... [with] muzzle velocity of about 3,500 feet per second normal and 4,800 feet per second with the tungsten carbide projectile". The T26 has been sufficiently future-proofed, according to the author. "These development programs will provide the American Army again in 1945 with the most powerful and best tank on the battlefield, and should the war continue, will provide our troops with a still more powerful tank in 1946."