The Red Signal — Grace Livingston Hill

When Hilda Lessing discovers that spies are planning to blow up a bridge, she is desperate! Such an explosion would destroy top secret cargo being transported over the brige ... and endanger engineer Dan Stevens, a man who once saved Hilda's life. Now Hilda must risk everything in a heroic battle to save Dan from certain death in a dangerous plot of intrigue.

Will she be able to reach Dan before it is too late?