Is it possible to achieve a 100% transported percentage?

Solution 1:

According to an entry related to industry transportation percentage:

Industries may decide to keep back part of the production and not deliver it to any station.

...which may account for the lack of perfection. (Not that 93% is worth complaining about, mind you.)

On a related note (based on the answers to this question) it would seem goods that are still in transport aren't considered delivered and thus are not counted towards the transportation percentage. Since the percentage is calculated for a month, that may account for the difference (when the end of the month arrives and there are still goods on the track.)

The calculation mechanics can be found here, as well as a (currently nonoperational) link to the actual code making the calculation.

Solution 2:

In order to achieve a 100% transported percentage, you need a 100% (perfect) station rating for a cargo.

Talking about the station rating:

You need 255 rating-points to get to perfect. Let's take a look at all the sources:

• Vehicle speed -- 42 points for 255 kmh-ish (0 points at 85 kmh-ish, 4 pt per kmh-ish above it).
• Vehicle age -- 30 points for <1 year.

Typically not financially speaking worth it to achieve this. (Having to buy new trains every year).

• Days since last pickup -- 130 points if there's always a train loading cargo.
• Amount of cargo waiting -- 40 points if all cargo is being picked up.
• Statue in town -- 26 points.
• Ad campaign -- 160 points temporarily.

Of these sources, 238 points are easy to get (everything except vehicle age and ad campaign), which is why you see 93% in the modern era. With early steam engines, you would see 76%, missing about 17% in vehicle speed effect.

If you want those last 17 points, there are two alternatives, both of which involve constant micromanagement.

• Option 1: Put your loading stations at most 20 tiles away from a town center. Pause and press 'fund large advertising campaign' every week or so.
• Option 2: Replace your locomotives when they are 11 months old. Unfortunately this cannot be automated (the autorenew option only works at 1 year before max age). So this involves manually sending each train to the depot and rebuying it at the end of every year. One way of automating this is to 'downgrade' (using autoreplace) your vehicles to the 2nd best engine at the end of each year, then upgrade it to the best engine for next year, and keep cycling.

    Year 1: Best engine. 
    Year 2: 2nd best. 
    Year 3: Best engine
    Year 4: 2nd best

Another way

You can use this newGRF. It makes the algorithm for station ratings more lenient and less dependent on vehicle speed. It's very nice for using together with complex industry sets like FIRS or ECS.