Transportation Secretary Brock Adams said yesterday that he intends to "substantially shorten" the deadlines by which owners of rail tank cars will have to retrofit the cars with safety devices.

In letters to Sen. James Sasser (D-Tenn.) and Lawton Chiles (D-Fla.), Adams said he was recommending the action because of the findings of hearings held earlier this month by the department into the causes of recent rail accidents involving hazardous materials.

Both senators are from states that have experienced fatal derailments involving hazardous materials in recent months.

In those hearings, federal rail safety experts said that tank cars should - and could - be fitted with better couplers and safety shields faster than the present federal timetable for such retrofitting.

The present regulation, which Adams now plans to amend, calls for all tank cars to have the safer couplers installed by July 1, 1979, and safety shields and thermal protectors by Jan. 1, 1982.

Adams said that he will propose "a substantial shortening of the retrofit period" by May 9, although he did not indicate how much time he would ask to be cut from present deadlines.

In response to the announcement, Jack Kruizenga, president of the Union Tank Car Co. of Chicago, said that the Railway progress Institute Tank Car Steering Committee "is anxious to comply with the most reasonable rule the Federal Railroad Administration can prescribe. Indeed, we have testified that we would meet an accelerated schedule."