Mod: Resources require route

As a test project, I thought I'd try to make it so that the Horse resource required a road before you had access to Horses.

I have been able to alter the tile yield based upon the presence of a road. There is a table called "Improvement_RouteYieldChanges" which has this effect. I verified this functionality using the following code:

Spoiler :

Code:

<GameData>
	<Improvement_RouteYieldChanges>
		<Row>
			<ImprovementType>IMPROVEMENT_FARM</ImprovementType>
			<RouteType>ROUTE_ROAD</RouteType>
			<YieldType>YIELD_GOLD</YieldType>
			<Yield>10</Yield>
		</Row>
		<Row>
			<ImprovementType>IMPROVEMENT_TRADING_POST</ImprovementType>
			<RouteType>ROUTE_ROAD</RouteType>
			<YieldType>YIELD_SCIENCE</YieldType>
			<Yield>1</Yield>
		</Row>
	</Improvement_RouteYieldChanges>
</GameData>

Using this code, a farm does yield an extra 10 gold after a road is placed to it, and the trading post does yield the extra science when it has a route.

However, I can't see any table which might alter the access to the strategic resource based upon a road to the improvement which accesses the strategic resource. The only thing I can change based upon roads are tile yields (eg. gold, production, etc.), and not strategic resource yields (horse, iron etc)

Can anyone help?

 

Code:

	<Improvement_ResourceTypes>
		<Row>
			<ImprovementType>IMPROVEMENT_MANUFACTORY</ImprovementType>
			<ResourceType>RESOURCE_IRON</ResourceType>
		</Row>
		<Row>
			<ImprovementType>IMPROVEMENT_MANUFACTORY</ImprovementType>
			<ResourceType>RESOURCE_COAL</ResourceType>
		</Row>
	</Improvement_ResourceTypes>

Makes the Manufactory both give you iron and coal, and also be buildable on top of it independent of terrain (but not feature). I don't think you can use routes with that though.