We found a small envelope in the archive car in June 2018 that was labeled:
Derailment at Belden MP 261 4/7/73
Belden April 7 73
It had 3 1/2 x 5 satin finish color prints typical of the mid-70's and negatives from three rolls of 35 film.
The negatives were scanned and numbered 1-23, 101-121 and 204-215. It is not known the real order of the three rolls of film.