All together now...

Telegrammed by Alfred J Prufrock
Question: What is the book life of trains?
Answer: 30-35 years

Question: So given an even spread from new through to about to be scrapped what would the average age be?
Answer: 15-17 years.

Of course this only applies if you are replacing 300 or so vehicles a year

Question: How many vehicles are DafT replacing a year?
Answer: None

In which case we're in a Hymns Ancient and Modern Situation.

Change and decay in all around I see.
Funder of the railways oh invest in me.