I finally resolved line 11 with the help of a friendly reader/critic. 'Ebbs' was the crucial word.  What happens if it never occurs to you - or if you have no friend on hand?  A thesaurus, I suppose might be the answer.

Anyway, this I feel is the OK version:

What kind of waiting is the worst? It depends.
You’re at the boarding gate. The flight’s delayed
Another hour – or two? You have no way
Of knowing when this frustration might end...
Or, the train’s come to another halt
Some way outside a station. No one knows why.
The carriage seems to heave a communal sigh:
It doesn’t help to think it’s no one’s fault.
But worse – the waiting that caps everything
Is open-ended - drags on and on. Hope
Ebbs but never fully drains away.
It’s not knowing if it may be today
The daily post will bring the envelope,
Not knowing when – or if – the phone will ring.

Some decisions made.  But some parts still not satisfactory:

But worse – the waiting that caps everything
Is open-ended. Drags on and on. Hope
Drains / declines (?) but never fully goes away.
It’s not knowing if it may be today
The daily post will bring the envelope,
Not knowing when – or if – the phone will ring.

The first section has taken shape.  Line 6 seems to work and is based on experience:

What kind of waiting is the worst? It depends.
You’re at the boarding gate. The flight’s delayed
Another hour – or two? You have no way
Of knowing when this frustration might end...
Or, the train’s come to another halt
Some way outside a station. No one knows why.
The carriage seems to heave a communal sigh:
It doesn’t help to think it’s no one’s fault.

The sonnet length won't allow me space to consider pleasant anticipation. Another reason to break away from my addiction to it perhaps.  Or a way of ensuring succinctness?  So the sestet focuses on the worst kind of waiting - when you don't know if it will ever end.
…
Or if the letter has yet to be sent / was ever sent
Not knowing when – or if – the phone will ring.

The post will bring the envelope

How to start?  The kind of waiting that caps everything…
or
And worse – the waiting that caps everything
Is open-ended. Drips on and on.  Hope
Drains away but never disappears
It’s not knowing ….
         …. If today’s post will bring the envelope
Not knowing when – or if – the phone will ring.

But worse – the waiting that caps everything
Is open-ended. Drags on and on. Hope
Drains (?) but never fully goes away.

For you never know / it’s not knowing      if it may be today
The (daily? – repetition…) post will / may bring the envelope
Not knowing when – or if – the phone will ring.

Lots of questions but beginning to feel some answers too.

Snow. Can't go
Anywhere, so...

Let's see if we can knock these first lines into sme kind of shape.

Add 'it depends' to the first line and change line 4 to give me my most frequent rhyme scheme ABBA - followed by CDDC.

What kind of waiting is the worst? It depends...
You’re at the boarding gate. The flight’s delayed
Another hour. You’ve no way
Of knowing when this ..... might end.

Or The train’s come to another halt
……………………………………..No one knows why
The carriage seems to heave a communal sigh
It doesn’t help that it’s no one’s fault

Which creates an octet I'm reasonably happy with.

Oh yes, it's going to be another sonnet - didn't I mention that?
Snow. Can't go
Anywhere, so...
Think on
Write a son
net

But that reminds me
The open ended wait’s the worst
You’re at the boarding gate. The flight’s delayed one more hour
The train’s come to a halt – no one knows why
The doctor probably will see you – join the queue

And worst of all – not knowing when the car will turn into your street
Or when or if the phone will ring (or beep or jingle)

Other related thoughts, jottings...
The show will go on…the lights eventually will dim
Sitting in the bar or café wondering if they or you have mistaken the time or date

First try at a few lines that cohere:

What kind of waiting is the worst?
You’re at the boarding gate. The flight’s delayed
Another hour. You’ve no way
Of knowing how long this might go on
The train’s come to a halt
No one knows why
The carriage heaves a communal sigh
It doesn’t help that it’s no one’s fault

It led me to think about a scale, a continuum of waits...

What is the worst – let that rest -
The  best (will this be a poem that has a rhyme?)
The best is the wait – the anticipation of some thing good – a holiday, the visit of a friend, a meal whose taste you already smell
Or even the anticipation of your favourite TV programme

Standing on a platform’s not too bad
Waiting for the 8.55 from Barnetby
Or home, from London, Sheffield, Doncaster
Is not so hard– the train does come, mostly on time

There will be a bus, a cab, a plane….   well, almost always.

Having just been to the dentist I was thinking about the activity of waiting. I remember waiting at the doctor's with my mother when I was little. It seemed to be a very frequent occurrence. Is there anything more mournful than a 'waiting room'?  A room set aside for waiting?

As an occupation, waiting is soul destroying - and I think it's because we often have absolutely no control over it. I was also, recently, awaiting a phone call and that, too, is frustrating.  It's the open-ended nature of it which makes it hard to bear.  I try not to think of those tormented parents waiting, hoping for a call from a missing child.
Is there an occupation more deadly than a wait?
Is how I began.  Born out of a wait for a phone call and then a general rush of thoughts and emotions about the feeling, the notion of waiting.

So I've started jotting down thoughts for a poem on the subject.