Version: Quark-r2.4-281
Minecraft: 1.16.4
Enigmatica6 - 0.3.2

Running into the following when using other mods redstone controlled rails:

Useful Rails - Holding Rail
Transport - Switcher Rail

Easiest to replicate with Useful Rail's Holding Rail - when the block is powered by redstone, the crash will happen as soon as a mine cart touches it. Let me know if more information is needed. The following log is from testing with Transport's Switcher Rail

[08Jan2021 21:51:19.215] [Server thread/ERROR] [net.minecraft.server.MinecraftServer/]: Encountered an unexpected exception
java.lang.NoClassDefFoundError: vazkii/quark/automation/base/ChainHandler
at xyz.brassgoggledcoders.transport.compat.quark.QuarkConnectionChecker.getLeader(QuarkConnectionChecker.java:21) ~[transport:1.16.4-3.3.0]
at xyz.brassgoggledcoders.transport.block.rail.turnout.AbstractSwitchRailBlock.shouldDivert(AbstractSwitchRailBlock.java:35) ~[transport:1.16.4-3.3.0]
at xyz.brassgoggledcoders.transport.block.rail.turnout.AbstractSwitchRailBlock.getRailDirection(AbstractSwitchRailBlock.java:92) ~[transport:1.16.4-3.3.0]
at net.minecraft.block.RailState.(RailState.java:25) ~[?:?]
at net.minecraft.block.AbstractRailBlock.func_208489_a(AbstractRailBlock.java:103) ~[?:?]
at net.minecraft.block.AbstractRailBlock.func_235327_a_(AbstractRailBlock.java:55) ~[?:?]
at net.minecraft.block.AbstractRailBlock.func_220082_b(AbstractRailBlock.java:50) ~[?:?]
at net.minecraft.block.AbstractBlock$AbstractBlockState.func_215705_a(AbstractBlock.java:610) ~[?:?]
at net.minecraft.world.chunk.Chunk.func_177436_a(Chunk.java:283) ~[?:?]
at net.minecraft.world.World.func_241211_a_(World.java:199) ~[?:?]
at net.minecraft.world.World.func_180501_a(World.java:176) ~[?:?]
at net.minecraft.item.BlockItem.func_195941_b(BlockItem.java:149) ~[?:?]
at net.minecraft.item.BlockItem.func_195942_a(BlockItem.java:54) ~[?:?]
at net.minecraft.item.BlockItem.func_195939_a(BlockItem.java:39) ~[?:?]
at net.minecraftforge.common.ForgeHooks.onPlaceItemIntoWorld(ForgeHooks.java:608) ~[forge:?]
at shadows.apotheosis.deadly.asm.DeadlyHooks.onItemUse(DeadlyHooks.java:121) ~[apotheosis:4.4.1]
at net.minecraft.item.ItemStack.func_196084_a(ItemStack.java) ~[?:?]
at net.minecraft.server.management.PlayerInteractionManager.func_219441_a(PlayerInteractionManager.java:350) ~[?:?]
at net.minecraft.network.play.ServerPlayNetHandler.func_184337_a(ServerPlayNetHandler.java:958) ~[?:?]
at net.minecraft.network.play.client.CPlayerTryUseItemOnBlockPacket.func_148833_a(SourceFile:36) ~[?:?]
at net.minecraft.network.play.client.CPlayerTryUseItemOnBlockPacket.func_148833_a(SourceFile:10) ~[?:?]
at net.minecraft.network.PacketThreadUtil.func_225383_a(SourceFile:21) ~[?:?]
at net.minecraft.util.concurrent.TickDelayedTask.run(SourceFile:18) ~[?:?]
at net.minecraft.util.concurrent.ThreadTaskExecutor.func_213166_h(SourceFile:144) ~[?:?]
at net.minecraft.util.concurrent.RecursiveEventLoop.func_213166_h(SourceFile:23) ~[?:?]
at net.minecraft.server.MinecraftServer.func_213166_h(MinecraftServer.java:733) ~[?:?]
at net.minecraft.server.MinecraftServer.func_213166_h(MinecraftServer.java:159) ~[?:?]
at net.minecraft.util.concurrent.ThreadTaskExecutor.func_213168_p(SourceFile:118) ~[?:?]
at net.minecraft.server.MinecraftServer.func_213205_aW(MinecraftServer.java:716) ~[?:?]
at net.minecraft.server.MinecraftServer.func_213168_p(MinecraftServer.java:710) ~[?:?]
at net.minecraft.util.concurrent.ThreadTaskExecutor.func_213161_c(SourceFile:127) ~[?:?]
at net.minecraft.server.MinecraftServer.func_213202_o(MinecraftServer.java:696) ~[?:?]
at net.minecraft.server.MinecraftServer.func_240802_v_(MinecraftServer.java:645) ~[?:?]
at net.minecraft.server.MinecraftServer.lambda$startServer$0(MinecraftServer.java:232) ~[?:?]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_242]
Caused by: java.lang.ClassNotFoundException: vazkii.quark.automation.base.ChainHandler
at java.lang.ClassLoader.findClass(ClassLoader.java:524) ~[?:1.8.0_242]
at java.lang.ClassLoader.loadClass(ClassLoader.java:419) ~[?:1.8.0_242]
at cpw.mods.modlauncher.TransformingClassLoader.loadClass(TransformingClassLoader.java:106) ~[modlauncher-8.0.6.jar:?]
at java.lang.ClassLoader.loadClass(ClassLoader.java:352) ~[?:1.8.0_242]
... 35 more

You should report this to Transport, they're still using an old class name that needs to be updated.