[amp_mcq option1=”On Single line” option2=”On Double line” option3=”Single/Double line both” option4=”None of these” correct=”option3″]
The correct answer is: C. Single/Double line both.
A block back is a type of railway switch that is used to divert trains from one track to another. It is typically used on single-track lines, where it allows trains to pass each other safely. However, it can also be used on double-track lines, where it can be used to create a temporary single-track section for maintenance or other purposes.
Option A is incorrect because block backs can also be used on double-track lines. Option B is incorrect because block backs can also be used on single-track lines. Option D is incorrect because block backs are used in both single- and double-track lines.