split_at_index where index=1

Splitting at index=1 results in multiple empty trains being created.
Can be prevented by an atc_speed > 0 check by the user, but easier just to return false and do nothing.

advtrains_luaautomation/README.md

@@ -260,6 +260,8 @@ There are several functions available especially for shunting operations. Some o
  - `split_at_index(index, atc_command)`
 	Splits the train at the specified index, into a train with index-1 wagons and a second train starting with the index-th wagon. The `atc_command` specified is sent to the second train after decoupling. `"S0"` or `"B0"` is common to ensure any locomotives in the remaining train don't continue to move.
 	
+	`index` must be more than 1 to avoid trying to decouple the very front of a train.
+	
 	Example: train has wagons `"foo","foo","foo","bar","bar","bar"`  
 	Command: `split_at_index(4,"S0")`  
 	Result: first train (continues at previous speed): `"foo","foo","foo"`, second train (slows at S0): `"bar","bar","bar"`

advtrains_luaautomation/atc_rail.lua

@@ -56,7 +56,7 @@ function r.fire_event(pos, evtdata, appr_internal)
 		split_at_index = function(index, cmd)
 			if not train_id then return false end
 			assertt(cmd, "string")
-			if type(index) ~= "number" then
+			if type(index) ~= "number" or index < 2 then
 				return false
 			end
 			local new_id = advtrains.split_train_at_index(train, index)