Complex sentence: In the 1850s proposals to build a transcontinental failed because of the disputes over slavery in Washington; with the secession of the Confederate states in 1861, the modernizers in the Republican party took over Congress and wanted a line to link to California.
Split into simpler sentences: In the 1850s proposals to build a transcontinental failed because of Congressional disputes over slavery. With the secession of the Confederate states in 1861, the modernizers in the Republican party took over Congress and wanted a line to link to California.