Feature spec
Goal: Send a clear invoice after finishing a job.
- Add a client, line items, a due date, and payment instructions.
- Preview the total before exporting a PDF.
- Keep an editable draft if export fails.
App Overview
Highball: Train Spotter
Highball shows you exactly when trains are arriving near you
$1.99 · In-app purchases
PRICE
$1.99
In-app purchases
BUSINESS MODEL
Paid
No overall Grossing estimate available
RATING
N/A
No ratings yet
Evidence scope: No current overall US chart position is available. Category-chart positions shown in discovery are a separate scope. Revenue is a directional model estimate, not verified earnings.
Build your own version
Build Kit
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A product spec, screen plans, and build steps for your coding agent.
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What's included16 files
A product spec, phased build plan, differentiation notes, and screen references.
- START_HERE
- AGENTS
- EVIDENCE
- SPEC
- BUILD_PLAN
- DIFFERENTIATION
- skills/
- screens/
Preview a sample Build Kit No email needed
Illustrative example: an invoice app for independent contractors. Your kit is tailored to the app you choose; this is not its generated content.
Screen plan
Invoice editor: Client at the top, editable line items in the middle, total and preview action at the bottom.
States: Empty draft, validation errors beside each field, exporting, and a retry action that preserves the draft.
Differentiate and validate
Hypothesis: Contractors need faster repeat invoices more than more templates.
First experiment: Test duplicating a previous job with five contractors. Watch where they hesitate before expanding the feature set.
The full kit adds a phased build plan, evidence notes, tool prompts, and implementation guidance. It is a plan for your coding agent, not a finished app.
Usage: Mechanics are fair game; never reuse the original name, branding, assets, or verbatim copy.
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Free quick prompt For a smaller starting point
Quick prompt
Good fit for mobile-first iOS and Android prototypes with managed app scaffolding.
// RORK SYSTEM PROMPT
Rork plan preview for Highball: Train Spotter: map the core Travel workflow, choose the smallest differentiated feature set, define the data and monetization boundaries, then prototype the riskiest user journey first.
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Opportunities
AI-GENERATED OPPORTUNITY HYPOTHESES
These are generated suggestions, not verified review quotations. Check current App Store reviews and speak with users before treating a possible gap as a requirement.
Hypothesis 1
Implement advanced filtering and sorting options.
Assumption to check: Difficulty in finding specific train sightings
Validate with people who use this workflow before building.
Hypothesis 2
Add user profiles and social sharing capabilities.
Assumption to check: Limited user engagement features
Validate with people who use this workflow before building.
Hypothesis 3
Integrate live tracking and notifications for train sightings.
Assumption to check: Lack of real-time data updates
Validate with people who use this workflow before building.
Hypothesis 4
Create a forum or comment section for users to share spotting experiences.
Assumption to check: No community aspect for sharing experiences
Validate with people who use this workflow before building.
App evidence
Original App Store material. Ratings and screenshots describe the existing app, not proof of demand for your version.
App Store description
Highball shows you exactly when trains are arriving near you. Whether you're a railfan, a parent with a kid who loves trains, or just standing near the tracks wondering what's coming — Highball puts live train data in your pocket. LIVE TRAIN TRACKING See every train operating near you with real-time arrival countdowns. The hero timer shows the next train at a glance — no timetables to decode. INTERACTIVE MAP Watch trains move in real time with directional arrows showing which way they're heading. Tap any train for route details, speed, and ETA. When multiple trains overlap, swipe through the chip bar to select one. SPOT LOG Spot trains you see and build your collection. Every spot records the train number, route, time, and your location. Earn badges as you go — from Rookie to Train Master. NOTIFICATIONS Get alerted when a train is approaching within 15 minutes. Works even when the app is in the background so you never miss one. SUPPORTED RAILROADS • Metrolink (Southern California) • Caltrain (San Francisco Bay Area) • More agencies coming soon FEATURES • Live GTFS-RT data refreshed every 30 seconds • Bearing inference shows train direction even when feeds don't provide it • Pull-to-refresh on every screen • Persistent spot log that survives app restarts • English and Spanish localization • Accessibility labels throughout • No account required — just location Highball is named after the historic railroad signal that meant "all clear — full speed ahead."
Written user reviews are not shown here. Check current App Store reviews >
Screenshots
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Build scope
Plan a focused first version with your coding agent. These are planning assumptions, not a delivery guarantee.
Define the first version
Choose one audience and one core workflow. Use the kit to agree on its screens, data, and acceptance criteria before building.
Set boundaries
Decide which secondary features, integrations, and platform support can wait. Your version does not need to reproduce everything in the original.
Check the hard parts
Validate external services, specialist technology, data access, and ongoing costs for your chosen scope.
SUGGESTED BUILD STACK
React Native
Expo
Supabase
react-navigation
react-native-maps
Zustand
BUILD KIT DIFFICULTY
Intermediate
From the Build Kit assessment.
A reliable timeline needs an agreed scope and a technical check. Ask your agent to estimate the phases in BUILD_PLAN.md after that review.
