Lionel 3 rail Fastrack running MTH loco's

About 6 years ago I built my O gauge layout in my basement. It is all Lionel fastrack. I have two switches beside beside each other and when I run my MTH Big Boy steamers or almost all of my larger loco's when passing through the double switches if running slower than half speed, about 10 volts, the loco's either hesitate or if running slower, they will stop and usually shut down.  I have not been on this O  forum in quite a while but I hope someone out there can tell me what I need to do to correct this problem. I hope I made myself clear about the problem. I tried to include a photo of my switches in question on my layout. Thanks, The-Menace