when did ditch lights become manditory

[thirdrail1]

Ditch lights in the US became mandatory because Sen. Nancy Kassebaum of Kansas insisted on them, thinking they would reduce rail-highway crossing accidents. The dates ran through the 1990's, varying on new or old power, whether or not the railroad used mars lights or other safety lights, and what the size of the railroad was. The whole history was hashed out at www.trainboard.com a few months ago.

[Anonymous]

When and why did ditch lights become manditory, to would help me date photographs
thanx Chuck