Grace Livingston Hill 
The Red Signal 

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 bridge—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.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 bridge—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.