If they are both traveling with the same speed that means that they will reach other in the middle of the line initially between them. In other word, each will have to travel the same amount before they reach other.
Now you can calculate the time it takes for only one locomotive to travel half of the total distance between them, and that time is equal to the time you are looking for.
Use
t = S1/2 / v
where t-time, S-distance traveled , v-velocity