DescriptionField for train numbers

we need the functionality to define description fields for train number handling.

Therefore we propose to define a new element description field and to mark a tvdSection as description field.

Description fields we can have on track section, points branch or tip, derailer and crossing.

Special cases for description fields are horizontal and vertical stacking description fields.

The proposal is to define a new railML element with the following attributes:

· id

· masterDescriptionField (primary descriptionField, which is always used first and released last) (optional)

· pileNumber (int) (defines the order in the stack) (optional)

· reference to track (ref) (optional)

· orientation (enum: vertical/horizontal) (optional)

· deleteOnLeave (bool) (optional)

· border (bool) (optional)

Our proposal is to define the new description field element in the interlocking.

At the interlocking element tvdSection we need the possibility to refer to one or more description fields with the following attributes:

· reference to track element (switch, derailer, crossing, track section) (optional)

· leg

o for points: tip, left leg, right leg

o for crossings: A, B, C, D

o no other information is required for other elements (e.g. for track sections: empty)