Complex sentence: The modern route is based on that designed by Telford, but with a number of improvements primarily dating from the 1920s and 30s, including a diversion across Rannoch Moor and another around Loch Leven, which was subsequently replaced by the Ballachulish Bridge.
Split into simpler sentences: The modern route is based on that designed by Telford, but with a number of improvements primarily dating from the 1920s and 30s. These include a diversion across Rannoch Moor and another around Loch Leven, which was subsequently replaced by the Ballachulish Bridge.