The Thoroughbred Board of the Maryland Racing Commission is expected to approve today a 1983 racing schedule that eliminates the concept of the 48-day summer meeting that had alternated among the state's three mile tracks, it was learned last night.

But there still will be 306 thoroughbred racing dates, the same number as in recent years. The new schedule would not affect the 42-date meeting at Timonium Race Course, the lone remaining track among the so-called "half-milers."

The significance of the new schedule, which will be in effect through 1985, is that it is the first time that management of the three mile tracks--Laurel, Pimlico and Bowie race courses--have agreed on dates. Under the new schedule, Bowie will race 78 days during the winter, at the beginning and end of the year, and 34 days during the summer. The dates include 36 the track bought from now-defunct Marlboro Race Course.

The 1983 schedule, as expected to be approved by the commission at its regular meeting today at Bowie, will open Jan. 1 at Laurel, at the end of Laurel's current meeting, and then switch to Bowie for 62 dates, from Jan. 3 to March 15. That meeting will be followed by Pimlico's 76 dates for the year, from March 16 through June 11.

Bowie will reopen June 13 for 34 racing dates and close on July 26, with four dark days. The 42-day Timonium meeting starts the next day and is scheduled to conclude on Sept. 13. Laurel opens the following day and is scheduled to race 75 dates without dark days, closing Dec. 9. The year's final 16 dates will be raced at Bowie starting the next day. There will be dark days on Dec. 23 and 24 and one other day to be announced.

It could not be learned whether Timonium will be allowed to conduct two dates on Sundays during the state fair, as the commission allowed for the first time this year.

The new schedule will benefit the tracks by resulting in a more constant annual cash flow, said sources close to the racing commission and Chick Lang, Pimlico's general manager. Until this year, when the commission forced Laurel to make improvements and play host to the summer meeting, those 48 dates had been run in alternating years at Bowie and Pimlico with the three tracks sharing profits or losses.