THE PLATEAU of trailer shipments continued in April-at historically high levels. Trailer manufacturers shipped an estimated 23,361 complete trailers during April, according to preliminary estimates published by the US Bureau of Census. The figure is very much in line with what output has been each month of 1999. After a low of 22,893 complete trailers in January, shipments have not varied more than 675 trailers between February and April. March has been the high point so far in 1999 with shipments of 23,905 trailers. Shipments edged down 2% in April from the March high. Almost every type of trailer was relatively unchanged compared with the previous month. Only insulated vans (down 10%), livestock vans (up 19%) and pole and logging trailers (off 17%) posted more than a single-digit difference from the shipments posted in March. In addition to the 23,361 complete trailers shipped in April, manufacturers turned out an estimated 1,978 containers and container chassis during the month, down 1% from March. The industry also shipped an estimated 258 dollies and converter gear during the same period, off 9% from March. During the first four months of 1999, the industry turned out an estimated 93,389 complete trailers. If shipments continue at the current reported rate, the industry will turn out 280,167 for all of 1999-a performance that would be at or near the best year on record for the industry. Officially the industry's best year was in 1995 when manufacturers shipped 279,144 complete trailers. However, the industry may have set a record last year. The US Bureau of Census is expected to publish a report on 1998 trailer shipments this summer. Truck Sales Increase Truck sales continued strong in April, edging up 4.2% from April 1998, according to figures compiled by Wards Communications. The gains were fueled by medium and heavy trucks, the sales of which were up more sharply than light-duty models. Class 1 trucks (GVW ratings of 6,000 pounds or less) slipped less than 1% in April, the only decline among the eight GVW classes. However, the 1,572,747 Class 1 trucks sold during the first four months of 1999 represented a 7% increase from the year-to-date sales of 1998. Class 2 trucks (GVW ratings of 6,001 to 10,000 pounds) were up 10% for the month and 16% year to date. Dealers sold 196,968 Class 2 trucks in April and a total of 727,664 for the first four months of 1999. Class 3 trucks (GVW ratings of 10,001 to 14,000 pounds) were up 18% in April, as dealers sold 10,945 units during the month. The 38,877 Class 3 trucks sold during the first four months of 1999 were 39% more than the 27,967 sold during the corresponding period last year. Class 4 trucks (GVW ratings of 14,001 to 16,000 pounds) increased 21% for the month and 22% year to date. Customers bought 4,970 Class 4 trucks during April, resulting in 16,280 Class 4 trucks being sold during the first four months of this year. Class 5 trucks (GVW ratings of 16,001 to 19,501 pounds) had a 19% sales increase for the month and were 68% above sales for the first four months of 1998. Dealers sold 2,609 Class 5 trucks during April and 10,961 during the first four months of 1999. Class 6 trucks (GVW ratings of 19,501 to 26,000 pounds) had sales of 3,656 units during April, up 22% from April 1998. The 13,303 Class 6 trucks sold during the first four months of 1999 were 41% more than were sold during the corresponding period last year. Class 7 trucks (GVW ratings of 26,001 to 33,000 pounds) had a 20% sales increase for April and were up 26% from the first four months of 1998. Totals were 11,862 and 45,878 Class 7 trucks for April and year to date, respectively. Class 8 trucks (GVW ratings above 33,000 pounds) had an exceptionally strong month-up 32% for April and 28% for the year. Dealers sold 24,332 Class 8 trucks during the month and 81,392 for the first four months of 1999. In another areas, intermodal shipments continued to grow, according to figures compiled by the Association of American Railroads. During the first 21 weeks of 1999, US railroads transported 3,569,731 containers and piggyback trailers, up 2.6% from the corresponding period of 1998. The growth came in response to a 4.8% increase in container shipments. The number of piggyback trailers shipped fell 1%.