Complex sentence: Promontory Summit had been agreed upon as the point where the two railheads would officially meet, following meetings in Washington, D.C. in April 1869, where it was also agreed that a ceremony would be held to drive in the Last Spike to commemorate the occasion.
Split into simpler sentences: Promontory Summit had been agreed as the point where the two railheads would officially meet following meetings in Washington, D.C. in April 1869. A ceremony would be held to drive in the Last Spike to commemorate the occasion.