In the given question, we are asked to identify which type of substation is installed on a high-voltage transmission line to improve network reliability by isolating faults without changing voltage levels.

The correct answer is the Switching Substation. Let's explore the reasoning behind this:

• Switching Substation: This type of substation is used to switch power lines. It connects or disconnects different parts of the network without changing the voltage levels. Its primary function is to enhance the reliability of the electrical network by isolating faults swiftly and efficiently. This prevents the spread of the fault across the network and helps in maintaining a stable supply.
• Step-up Transformer Substation: This substation is utilized to increase the voltage level from generators (where voltage levels tend to be lower) to high transmission levels for efficient long-distance power transmission.
• Step-down Transformer Substation: This substation decreases voltage levels from high transmission voltages to lower levels suitable for distribution to end-users.
• Traction Substation: This is used to supply power to electric railways and is specialized for traction load requirements.

As per the question's requirement, since the task is to isolate faults without altering voltage levels, the Switching Substation is the most appropriate and strategically beneficial choice.