Two tractor-trailor loads of packages, some mailed last October, were discovered Thursday at a rail yard in Southwest Washington and rushed to the Merrifield postal center for sorting and delivery.

Postal workers spent the night attaching letters of apology to the parcels, many of them Christmas presents, which were postmarked in Detroit and destined for addresses in Washington, Maryland and Virginia.

Postal officials said they did not know how long the vans had been sitting at the railyard at South Capitol and I Streets SW or why.

"We're giving it preferential handling," said the manager of the northern Virginia Sectional Center in Merrifield, where the last of the undetermined number of packages were being processed yesterday.

A postal service information officer, Jesse B. Brown, said that since Sept. 15, the postal service has used about 300,000 vans to handle the heavy mailings created by the United Parcel Service strike and the Christmas season. He said the two vans discovered Thursday were the only ones reported to have been delayed.

The trailers were transported by train to the Southwest yard, where they were supposed to have been attached to trucks and delivered to the Merrifield postal center. The railyard regularly serves as a temporary storage place for trailers, some of which are empty, and postal workers speculated that this might have caused the mix-up.

The letter of apology accompanying each delayed package stated that during the past few months, the postal service "handled millions of additional parcels because of the United Parcel Service strike and the usual heavy Christmas mailings. Sometime during that period, your parcel . . . was inadvertently delayed. Please accept our sincere apologies."