type
|
type: "A" to "E"
|
year
|
year of construction: 1960-64, 65-69, 70-74, 75-79
(coded as "60" , "65" , "70" , "75" )
|
period
| period of operation : 1960-74, 75-79 |
service
| aggregate months of service |
incidents
| number of damage incidents |