The design consists of the visual features of shape, ornamentation and pattern of the ADAPTER PAD FOR RAILCAR TRUCK. The portions shown in stippled lines form no part of the design.In FIGS. 8-14, the broken lines indicate indeterminate length and width and form no part of the design.IN FIGS. 11-12, the design has a constant cross-section between the break lines.FIG. 1 is a top, front, right perspective view of an ADAPTER PAD FOR RAILCAR TRUCK showing a first variant of our new design;FIG. 2 is a top view thereof;FIG. 3 is a bottom view thereof;FIG. 4 is a front view thereof;FIG. 5 is a rear view thereof;FIG. 6 is a right side view thereof;FIG. 7 is a left side view thereof;FIG. 8 is a top, front, right perspective view of an ADAPTER PAD FOR RAILCAR TRUCK showing a second variant of our new design;FIG. 9 is a top view thereof;FIG. 10 is a bottom view thereof;FIG. 11 is a front view thereof;FIG. 12 is a rear view thereof;FIG. 13 is a right side view thereof; andFIG. 14 is a left side view thereof.