You have to
add the Route in XML/Misc/CIV4RouteInfos like this (the italics is stuff you will have to replace because I made it myself and haven't posted it yet)
<RouteInfo>
<Type>ROUTE_TRACK</Type>
<Description>Cart Path</Description>
<iValue>1</iValue>
<iAdvancedStartCost>5</iAdvancedStartCost>
<iAdvancedStartCostIncrease>0</iAdvancedStartCostIncrease>
<iMovement>60</iMovement>
<iFlatMovement>60</iFlatMovement>
<BonusType>NONE</BonusType>
<PrereqOrBonuses/>
<Yields/>
<TechMovementChanges/>
<Button>Art/Interface/Buttons/Builds/
BuildPath.dds</Button>
</RouteInfo>
Be attentive to the Value tag. That's what order they replace each other in.
add a new set of art for the new route in
XML/Art/CIV4RouteModelInfos (you copy an existing route type in its entirity, and change the name in every one of these that currently has the name of what you copied)
<RouteModelInfo>
<ModelFile>Art/Terrain/Routes/Roads/RoadA00.nif</ModelFile>
<LateModelFile>Art/Terrain/Routes/Roads/RoadA00.nif</LateModelFile>
<ModelFileKey>A00</ModelFileKey>
<Animated>0</Animated>
<RouteType>ROUTE_DIRTROAD</RouteType>
<Connections>-</Connections>
<ModelConnections>-</ModelConnections>
<Rotations>0 90 180 270</Rotations>
</RouteModelInfo>
(This works because the only thing my Dirt Road route changes is the dds while using the Road Nifs)
To XML/Units/CIV4BuildInfos add a build for the route like this:
<BuildInfo>
<Type>BUILD_DIRTROAD</Type>
<Description>TXT_KEY_BUILD_DIRTROAD</Description>
<Help/>
<PrereqTech>NONE</PrereqTech>
<iTime>150</iTime>
<iCost>0</iCost>
<bKill>0</bKill>
<ImprovementType>NONE</ImprovementType>
<RouteType>ROUTE_TRACK</RouteType>
<EntityEvent>ENTITY_EVENT_SHOVEL</EntityEvent>
<FeatureStructs/>
<HotKey>KB_7</HotKey>
<bAltDown>0</bAltDown>
<bShiftDown>0</bShiftDown>
<bCtrlDown>0</bCtrlDown>
<iHotKeyPriority>0</iHotKeyPriority>
<Button
>,Art/Interface/Buttons/Units/Worker.dds,Art/Interface/Buttons/Charlemagne_Atlas.dds,7,2</Button>
</BuildInfo>
and to XML/Units/CIV4UnitInfos find worker and add a build like this:
<Build>
<BuildType>BUILD_DIRTROAD</BuildType>
<bBuild>1</bBuild>
</Build>
Any additional art must be included in the art folder with a path like this
Future106/Assets/Art/Terrain/Routes/Roads/
RoadPrimitive.dds
Heres a little mod with 6 routes.
http://forums.civfanatics.com/downloads.php?do=file&id=11872