Your xml isn't valid. Replace
</Outbound>
<TwoWheeler></TwoWheeler>
with
</FourWheeler>
The XML represents a single Vehicles node not a list of them so change the code accordingly
XmlSerializer xmlSerializer = new XmlSerializer(typeof(Vehicles));
System.IO.StreamReader sr = new System.IO.StreamReader( ... );
Vehicles flightavailabilitylst = (Vehicles)xmlSerializer.Deserialize(sr);
Also Availability is a list of Available objects, not a single Vehicles object so change that also
[XmlRoot("FourWheeler")]
public class FourWheeler
{
[XmlElement("Availability")]
public List<Availability> Availability
{
get;
set;
}
}