I did a little research on this event for an author who had heard the story anecdotally. I don't know if -Sgt. Charles Harrell, of Yuma, Ariz. and Pvt. Max Peterson, of Marlington, W. Va.,were actually in the 721st ROB but they were serving under General Joseph Stilwell on the Bengal-Assam line. Lots of additional various articles and information at link below

Kohima, India, May 27—
(Delayed)—(AP)—Two American soldiers, each with a three-day pass and a convincing line of sales talk, have brought a new version of the bushman’s holiday out of the Burma jungles today.

They spent their holidays killing Japanese.

“It took a little fixing,” said T-Sgt. Charles Harrell, of Yuma, Ariz.

“But it was worth it,” added Pvt. Max Peterson, of Marlington, W. Va., “yes, sir, quite an experience.”

They’re railroaders, attached to the special corps of American trainmen operating the Bengal-Assam line, which feeds the Allied armies in this area. They had never seen any combat.

So when they got their furloughs a short while ago they decided to go and find some.

It wasn’t as easy as it sounds. The nearest active front was in Manipur, where British troops are busily engaged in wiping out Japanese.

To go with the British, the two Americans obtained special permission from their own commanders, hitch hiked up the road from Dimapur, and kept going until they finally found a regiment of Scots.

They told the British commander what they wanted, produced their credentials, and went through the usual interrogation.

“We got a couple of good breaks, meeting that outfit,” said Peterson. “It was getting set for an attack. And it had tanks.”

Both men had had some experience in tanks. They set about talking their way into the operation. Armor, they said, was just their dish.

Probably more amused than convinced, the British commander finally consented.

The operation lasted 13 hours.

“And it was a dilly,” said Harrell. “These Scotch boys are tough guys.”

It seems the Japanese, pursuing their customary tactics, held their positions as long as any of them were still alive. The result was that both Harrell and Peterson actually saw the shells from their respective tanks as they crashed into enemy strongpoints barely 150 yards away. There were casualties in the crews of both tanks, too.

A day later, when the operation was finished, the two Americans went back to the more prosaic job of pushing freight up the railroad.

“Wouldn’t have missed it for anything,” they said.

More articles HERE 

721st history is here Rails of War  HERE 

War time editions of N.Y. Central Railroad Headlight are a goldmine and this edition includes mentions of soldiers in the units: 

CH Aug 1945

• 718thth ROB MRS Company B
• 714^th ROB Alaska
• 718^th ROB Luxembourg
• 701^st RGD
• 743^rd ROB Stimson
• MRS India

 War time editions of N.Y. Central Railroad Headlight are a goldmine and this edition includes mentions of soldiers in these units:

CH July 1945

• MRS India
• 758^th RSB Burma
• MRS Crane

War time editions of N.Y. Central Railroad Headlight are a goldmine and this edition includes mentions of soldiers in the units:

• 2nd MRS
• 721st Railway Operating Battalion
• 718th Railway Operating Battalion
• 774th Railway Operating Battalion

War time editions of N.Y. Central Railroad Headlight are a goldmine and this edition includes mentions of soldiers in the units: 

• 2nd MRS 
• Units in Burma, Italy, India 

Nov 1944 

• MRS India 

McHugh, Thomson, Donahy, and Mulaniff

June 1944 

• MRS Italy 
• RSB Italy Wottring, Melek, Trefz 
• India- Photos

May 1944 

• 718th Railway Operating Battalion 
• Camp Clairborne 
• Camp Polk RR Vigars 
• MRS India - Harvey, M.M. Emmanuel 
• 733rd Railway Operating Battalion 

Schoppes, Smith, Wright, Guest, Garey, Gander, Adis, Griffin, Baird, Griswold, Jorgenson, Bort, Yocum, Handler, Terpoilli, Panto, Seiglo, Anderson, Fulton, Guinano, Flam, Balsiger, Little 

721st-- 725th -- 726th

Lincoln: University of Nebraska / Potomac Books, 2017. Pp. xvi, 200. Illus., maps, notes, biblio., index. $29.95. ISBN: 161234853X.

  Military Railroading in Asia's Jungles and Mountains Inspired by a box of memorabilia left by his father, a sometime staff sergeant in the 721st Railway Operating Battalion (ROB), Hantzis set out to learn more about his father’s war, and the result is this interesting history of two largely overlooked subjects, military railroading and the neglected CBI theatre, which included some of the most inhospitable terrain in the world, the jungle clad mountains of northeastern India and Burma.
  After an introduction offering some family background and comment on military railroading, Hantzis covers the recruiting and organization of the ROB battalions, each of which was organized around a cadre drawn from one of the nation’s great railways, the 721st ROB was recruited from the New York Central, the 725th from the Chicago, Rock Island, and Pacific, the 726th ROB from the Wabash, and so forth. He then follows these troops on their nearly 70 day voyage aboard a converted former luxury liner from the East Coast to Bombay in India, and thence overland to the front in the northeastern part of India, altogether journey of nearly 20,000 miles. From there, the book follows these troops as the engaged in upgrading the local railroads, building new ones, and bringing up the matériel needed to support the Allied war effort in the theatre.
  As he tells the story of the organization, movement, and work of these troops, Hantzis also gives us a look at the battalion’s personnel and their individual experiences coping with alien cultures and war. In the process, he also fills the reader in on a lot of information about railroading, civil and military, so, for example, we learn that one ROB could sustain operations along a 100-150 mile stretch of line. Although his primary concern is the story of the 721st ROB, Hantzis sets this within the overall picture of events in the theatre. Rails of War will prove a profitable read for anyone interested in military railroading or the CBI. Note: Rails of War is also available in several e-editions

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