Why have passing loops not at platforms?

[Darrman]

So a single track railway needs places for trains to pass, and those places are usually the intermediate stations. This makes sense.

Sometimes in order to better space things out, passing loops are located in closed railway stations or other places a passenger can't get out at. I understand this.

What I don't understand are the cases where the passing loop is located in an operational station, yet the loop doesn't have a platform, rendering it useless to passenger service as the station has to be skipped. There's also cases where the loop is located slightly outside of the station, which doesn't prevent a call, but does result in a quick start and stop.

To my knowledge, Enfield, Clara and Tipperary have loops but no corresponding platforms, while Carrick-on-Shannon, Wexford, and Magheramorne's loops are located just off of the platform. What benefit is there for this, especially in the former situation?

[NIRCLASS80]

I suppose there are a number of factors at play. Here are a few. 
1. Some loops at stations may have only one platform due to the cost of maintaining another platform cannot be justified due to normal passenger traffic patterns but the loop can mean Permanent Way equipment and freight traffic can cross with the scheduled service. 
2. Sometimes loops are placed in strategic positions to help increase route capacity.  Belfast to Londonderry has a number of rural passing loops mainly used during the morning and evening rush hours to facilitate half hourly services. 
3. Platforms cost money to maintain so if not required on todays railway they won’t be their. 
 

Usually the biggest factor is traffic levels and planned schedules as loops are an extra expense. 

[airfixfan]

Magherabeg, Templepatrick, Killagan on the Derry Line all split sections but only Templepatrick is beside a populated area.

[David Holman]

In some ateas, freight is more important than passenger traffic, so a passing loop that allowed a freight train time and space to shunt, but still enabled a passenger train to pass or overtake, could be a good option.

 Lines with only light traffic didn't need every station to have a passing loop and some might just have a siding or two, while others might have a loop that was not signalled to pass trains, but instead was just a double ended siding that could be shunted from either end.

[Mayner]

Several stations have single platforms because there is/was no operational need to cross passenger trains at that particular station and it would be difficult to make the financial case for constructing a second platform that would meet current requirements (lifts, fencing, lighting without considering the cost of the civil works)

To the best of my knowledge the down or loop platform at Enfield was re-opened to cater for Longford/Mullingar commuter trains and Connolly-Sligo services run through without stopping on the Main Line. It would be difficult to see the NTA or Meath County Council funding a second platform if existing platform can handle current traffic needs.

The loop at Clara was originally a goods loop, passenger trains of the Portarlington-Athlone section of the Dublin-Galway-Westport route are regularly scheduled to cross at Geashill, Tullamore and Clonydonnin crossing loop (approx. mid way Clara-Athlone), though occasionally passenger trains cross at Clara.

There is/was a low Up platform at Tipperary though likely to have been out of use for many years. With a sparse passenger service (2 trains daily each direction)and CIE/IE wanting to close the line to passenger traffic from the late 70s it unlikely that IE/NTA or Tipperary County Council would be willing to fund an upgrade/new platform.

Wexford always had one platform Connolly-Rosslare passenger trains not regularly scheduled to cross at Wexford, the present crossing loop north of the station was installed in to 2000s when the line was upgraded to CTC operation. Wexford North was historically the terminus of the line from Dublin the line between Waterford and Wexford was originally operated by a separate company. Apart from Connolly-Rosslare passenger trains Wexford North was the terminus for Dublin-Wexford and Waterford-Wexford freight services and Rosslare Harbout-Wexford local passenger trains into the late 1970s.

Carrick-on-Shannon had a crossing loop and two platforms until the station ceased to be a block post and crossing loop removed as an economy measure during the late 1980s. The present crossing loop was installed when the Lines signalling system was upgraded to CTC operation during the early 2000s.

Reinstating the original up platform would have involved substantial and disruptive civil engineering works including widening the underbridge at the Dublin end of the station to accommodate the crossing loop and platform.