@schulz You essentially need each nation to have the mechanizedInfantry tech, each land transport have isLandTransport, and each transportable unit have isLandTransportable:
<attachment name="techAttachment" attachTo="Germany" javaClass="games.strategy.triplea.attachments.TechAttachment" type="player">
<option name="techCost" value="4"/>
<option name="mechanizedInfantry" value="true"/>
</attachment>
<attachment name="unitAttachment" attachTo="Train" javaClass="games.strategy.triplea.attachments.UnitAttachment" type="unitType">
<option name="isLandTransport" value="true"/>
</attachment>
<attachment name="unitAttachment" attachTo="germanInfantry" javaClass="games.strategy.triplea.attachments.UnitAttachment" type="unitType">
<option name="isLandTransportable" value="true"/>
</attachment>