Tim,
The following data was compiled from ACF bills of materials (BOM) covering cars built from 1931 to mid-1952 and available for review at the St. Louis Mercantile Library. Other data comes from some ACF drawings that apply to application of steel running boards.

The latest build date on a new ACF tank car delivered with wood running boards & dome platforms was 9-47 (lots 3118 and 3119). As was indicated in a previous message the wood thickness was a minimum of 11 3/4", but the thickness was often specified as 11 7/8" in the BOM. The majority of Type 27 tank cars built from the late-1920s to the immediate postwar period came with wood running boards & dome platforms, but steel was occasionally used.

The earliest application I have documented for any type of steel running boards was in March 1934 in which Allen Wood Diamond Plate (AWDP) was used on an order for DUPX (lot 1323). AWDP was used on a very limited number of cars (18) from 1934 to 1937 on 10 orders. In these cases the dome platforms were sometimes AWPD, sometimes wood, and in 3 cases subway grating.

From 1938 to 1947, more than 1,100 Type 27 tank cars received Allen Wood Super Diamond Plate (AWSDP) running boards and/or dome platforms on 31 orders. A few of these cars used wood dome platforms (6,000-gallon chlorine cars for SACX or PPGX). Three orders of USOX cars used Carnegie steel plate for the dome platforms. The latest applications of AWSDP running boards on an ACF tank car was CDLX 774-793, built 10-47 (lot 3050), and SHPX 3635-3699, built 9-47 (lot 3106).

Regarding the various types of open steel-grid running boards, Morton, Blaw-Knox, and Apex Tri-lok were all used on ACF Type 27 tank cars. Morton was used on just 2 orders for LSBX/LSPX (lots 2231, 2585) in 1941 and 1943, respectively.

Blaw-Knox (B-K) was first used on an ACF Type 27 tank car in 1941, RPCX 111-115, built 6-41 (lot 2220). The B-K running boards & dome platforms were not used for about 6 years until May 1947, at which time it was used more commonly through 1952.

Apex Tri-lok was used by RTCX as early as 4-41 (lot 2187). Apex was sporadically used on ACF tank cars built in 1942 to 1947 with approximately 1,500 Type 27 tank cars built during this period when wood was also being used.

Beginning in 9-47, all ACF tank cars (regardless of being called "Type 27" or not) received steel running boards and dome platforms. From all information I have seen, Apex and Blaw-Knox were the two types used starting in 10-47. Three ACF drawings that each apply to numerous lot numbers (dated 11-23-46, 10-28-48 and 11-22-49) indicate a 2" thickness for Apex running boards and 2 3/8" thickness for B-K.

A general comment about "Type 27" tank cars is that after World War II the term became somewhat nebulous and was inconsistently used by ACF in their lot number list and bill of materials documents. From 1946 to 1950, ACF tank car designs were in process of changing the dimensions of the underframe & tanks as well as a transition to welded underframes. The latest ACF cars specifying "Type 27" in the lot number list are lots 3392 (10 cars built 4-49) and lot 3409 (1 car built 10-48), however, a number of bills of materials after this continued to specify cars as Type 27 through lot 3485 (cars built 10-50). Anyhow, I hope this information helps.
Regards,
Ed Hawkins