Three trailers, Calm/u 500, 501 and 502, were delivered to Trenčianska elektrická železnica in 1954.

Later, they were designated Balm/u 649, 650 and 651.

In 1987, Balm/u 649 was converted into railcar EMU 46.1003.

In 1990-1991, 650 and 651 were converted to 911.901-7 and 911.902-5 driving trailers.

In 2008, 911.901-7 was transferred to Oravská lesná železnica.

In 2010, 911.902-5 was transferred to Čiernohronská železnica.